When you use photo reference you need to make sure that you have a legal right to use it. Someone who takes a photo has rigths to its usage. So just going on Google and finding a photo is not allowed in most cases.
Fortunately there are sites that offer free to use, royalty free images. A copule of sites to try are:

https://pixabay.com
https://unsplash.com/

When you are drawing from reference a key concept is to make sure you are looking at the reference often. Some will pull up a photo to draw but barely look at the photo. Since you are trying to draw what you see, you need to look at what you are drawing most of the time, rather than at your art.
Try to draw while you look at the reference. This is probably easier on paper at first, but you get accustomed to it. You want to focus on what you are trying to portray.
If you need to, have a friend watch you while you draw and give feedback. Did you look at the reference 10 percent of the time, 80 percent? Or did you only look at your art? Or record yourself and see how often you look at the reference.

Art is hard work. You need to protect against negative self-talk. One way to do this, and to improve at drawing, is to constantly ask questions aimed at what you need to do in order to better capture what you are seeing.
Instead of saying "this looks nothing like it", or "I really messed that up.". You can say, is that person's arm longer in the photo than how I drew it? Is that line more concave or convex? Is that shadow darker or lighter than how I portrayed it? Does her hand extend down to her knee in that photo, and does my drawing do the same? This kind of constructive dialogue helps you to get better instead of just getting frustrated.

You don't have to draw everything in your reference! You can leave things out. You can intentionally change things around to look better. Especially once you start to learn about composition you may want to intentionally change things in the painting to put the most attention on the main focal point.
It is often helpful to use multiple references for one piece of art. If you do be sure the lighting matches up in each photo as this is a key to getting it to look right. If they don't match up you have to use some brain-power to translate. Or sometimes you can flip one reference in a photo editing program to get them to match.

You can change your reference photo in a photo editing program before you paint. I like to adjust any lighting, contrast, crop to different view, move things around if need be, etc. before I start to paint, so that I am seeing it the way I want it to be. Even most smartphone apps can crop or add filters, etc.
And sometimes i just wind up doing a photomanipulation because that is fun too!

You can take your own reference images! Sometimes you can't find what you want, but you and your friends can make it. One tip in this regard is that if you want a convincing action reference it is better to video the action and then freeze it where you want, than to try to get people to convincingly fake frozen action.
Another great idea for this is to take a photo of a memorable event, day, etc. and then make a painting to remember that day.

Especially when you are first starting out finding reference with good contrast is helpful to learn how to paint shadows, lights, highlights, etc. It is hard to paint things that are flat. Make sure it has enough dark and light areas to see the forms.

If you are having trouble getting something to look right while using reference, try turning your reference upside down. Our mind categorizes things by characteristics. So we have an idea of what a face looks like a chair looks like, etc. But our mind can try to impose that as we draw, and keep us from seeing what is there. By putting the reference upside down we can sometimes overcome this.

When you are in the zone, painting from reference, you sometimes are not seeing your painting correctly. You get so into painting that it looks right, even when it is not. It helps to take a break at times.

It can also help to use the flip horizontal feature as this can often help you see the painting correctly and notice errors.
Standing back from the painting and seeing it at a distance on the screen can also help see errors, and see if the composition works well.

You can use a computer screen, or a phone, or even project your reference to the TV. I like to, when I can, sit in a comfortable chair and hold my 3ds/phone in my hand while looking at the computer monitor a distance away. It helps to be a little ways from your reference so you can see the overall composition and forms without getting fixated on details immediately. You can then zoom in when you need to look at details. This also means I can be painting and just glance up slightly to see the reference a ways away on the monitor, making it easy to focus on the reference while I paint.

When you paint from life you can squint to see a bit less detail when you are drawing the initial shapes.
If you have photoshop or gimp, or Krita, etc. you can use a noise filter to remove some of the detail when you first start your painting to get the same effect as squinting.

thanks for the tips. if i use a reference, and my finished art does not look abit like it, do i still quote it?

@Gray_GBC If you use royalty free you can check the terms. Many do not require attribution at all. You can still reference if you like. And some appreciate a link to see how it was used.

Another site with free to use images is https://commons.wikimedia.org/wiki/Main_Page

If you like specialized stock, especially for fantasy or sci-fi Deviant art has some good options. However, each artist sets their own terms there so be sure to check them. Some also do premium or even requests. One piece years ago that I did was the first ever use of stock for a model. And he commented on the piece when I linked it. He went on to do a lot of cool work and would check out works that used his stock. It can be more interactive. Of course, on DA be careful what you search.

thanks for sharing enormous information. these are essential and often been overlooked.

@Gay_GBC Sure!
It is not always clear to folks what images you can use and which you cannot. I am glad things have gotten a lot better now with sites designed for that.
Back in the old days you could draw from magazines, etc. but these were all copyrighted work.
I think the most important thing in all of these, however, is the basics of learning to see things, and to understand how light and shadow work, and how to then put that down on paper (or screen) while asking constructive questions.

One drill that really helps along with this is to select a difficult pose of a person and then draw it as best you can and show it to someone along with the reference. Yes, it will not match up. But that is the point. Ask that person to go through and mark everywhere that they don't match up. Then draw it again with that in mind. Show them again, have them mark again. If you do this a few times with various photos you will learn to look for what is off between the reference and your drawing, and how one part relates to anouther.
By doing this you are training your eye to see differences. And you are relying on someone who is not immersed in it to correct your misperceptions. This doesn't have to be an artist. People see so many other people every day that we know when something looks off. Though if you can find an artist even better.

We were discussing reference in a different part of the gallery. Sometimes people get discouraged just making poor copies of existing photographs. Drawing what you see in a reference is important practice. But the point of practice is to go beyond just reproducing photos.
A good goal is to improve upon any reference either in lighting, composition, etc. But that can also mean using multiple references.
Often times you can't find the exact reference you want for a particular painting. You may have to combine several. This also means the final outcome doesn't look like any one existing photo, which is a good thing. And you can interpret the parts you want of each reference.
Artists often use reference clouds, a way of arranging multiple references that have elements of what they want in the painting, that they can refer to during the process. Some like Pinterest for that. Others may stick them in photoshop and export them, etc.

In my entry for this contest #dtiysmimo I had to use five different references for the figure. I couldn't find any with the exact pose, so I got some that had parts of it. Some used a different direction of lihgting so I had to translate that. Some were more helpful for the hair, or the skin tone, or the folds in the clothing. And some had a lighting effect I wanted to try to incorporate. And I liked the result, but see a number of errors with it. But it is good practice. The result does not look like any one of the references, but is unique.
This is the reference cloud I used for that painting, if the link opens correctly:
https://cdn.discordapp.com/attachments/936380340430327857/941048137429168198/dresswatref_cloud.PNG

i dont quite understand how to use reference image...

@_K-u-r-o-k-o_ It took me a while to see this! You look at the picture, and draw what you see! You can draw from imagination, but the more you draw from a reference image the more you will start to know what things look like.

ohhh ok