Why did Bryan Adam's fingers bleed in the summer of '69?
Because every fricking guitar you can get near has a PAINFULLY HIGH ACTION (height of strings)
Guitars with sweet low actions do exist, but there's already a dude/ette with no biceps playing it 24 hours a day and going without food.
Here's how to lower the action of your own guitar.
It'll feel so good you'll play it instead of eating/bathing/sleeping.
There won't be anything left of you but bad posture and enough hair to comb over your face.
The cheap ones start out too high. Old ones get that way over time from the tension of the strings.
For examples of both kinds, I'll be dropping the action on this Chinese-made "Backpacker guitar" that cost me a penny brand new (plus $24.99 shipping on ebay) and also an original Martin Backpacker.
Step 1: Go Nuts on the Nut
You probably want your strings as low as they can be without buzzing while you play.
That depends a lot on how you play. If you play hard you'll need the strings to be higher or they'll buzz.
If you play slide or bottleneck guitar, you'll want the action higher still.
In fact, if you use the slide all the time, you don't really have to use the frets, and there's no such thing as an action that's too high. Keep that in mind when you get a free guitar with a really bent neck.
Regardless of your situation, the best way is to find a guitar you like playing, and measure the action
If you only have access to the one you're working on, just whittle down the notches in the nut and bridge until you like how low it is. If you wittle too low you can put a drop of superglue in the notch and let it dry. Or put a piece of paper under the string.
If your neck is straight and your fingerboard is flat, you'll be able to put them lower.
Here I am whittling down the nut notch on the penny guitar. You can use a triangular file if you have one.
Play with the string and sight down it. The nut notch should be as deep as if there were another fret there holding up the string. In other words the string height differece between the nut and first fret should be the same as the difference between the first two frets.