When I was starting out, one big mistake is that I make every stitch too tight. Loosen up your hands and wrists. When the stitch is too tight, one tends to push and pull too hard and is very tiring. And also, it will become deformed. When you push the hook thru, make sure it goes up the hook.
The size of the hook loosen the chain or loop for even stitch. That's something I learned from mistakes. No book ever told me how important it was to not just barely hook the yarn into the loop but go up the stem of the hook a bit.
Good luck.