How to make more space on my computer?

So I think this is a 2009 MacBook, it's the 13" one with the stupid rubber bottom that's peeled off because of the heat. This was passed down to me from my mom, I'm getting a new one soon but I need this to function for now. When I was looking at the "About This Mac" thing it says there are 2GB of memory. But.that's literally nothing. I mean. My phone has more space than that. 16 times more space. I've been able to back up my phone to iTunes in the past and such. After some searching I still couldn't find the total amount of data that it has the ability to hold. Everywhere I looked it just said 2GB.

Lately I've been having the problem where it says that the startup disk is almost full. I mostly encounter this problem when I'm drawing in Photoshop. I'll be drawing and then suddenly the notification will pop up. Sometimes I'll ignore it and I'll be fine but other times it will pup up again saying that the startup disk is full and I won't be able to draw another line or move a segment of the image or create or delete a layer. I don't understand why drawing lines would put a bunch of data on my computer? Today I backed up my computer and deleted over 4, 000 items. But I honestly don't have THAT much stuff on here. What I deleted was screenshots. I don't have nearly as many Photoshop files as my mother had either. After I complete an image I always save it as a png. And delete the one that was Photoshop. My mom also had zillions of photos in iPhoto. And deleted them all. I don't have that many in there now.

I haven't been able to update my iPhone because it's completely full, over 8, 000 photos on that thing. Why? Because there isn't enough space on my computer to download those photos. I have over a year's worth of photos stored on my phone. Like I said, I backed up my computer and deleted a bunch of things. I'm still hesitant to plug in my phone though because now that I have a bit more space, I'd LIKE to draw.

Honestly there really aren't that many programs or things on my computer that I know of. Is there some way that I can see what's taking up most of the space? What is filling up my start up disk? What are some definite unnecessary things that I can get rid of? This thing is pretty old, the hard drive could fail soon, it happened to my old computer and it just happened to my mom's computer which is currently getting a new hard drive and then it will be given to me. Is that what's happening to this thing?

I just need it to function. Does it really only possess 2GB of memory capacity? Or is that just what's available? What should I look for to delete?

Find some things that you don't need on your computer and delete them.
This includes unused shortcuts, documents, and programs that should make more space.