Welcome to knitting!!!
Please don't be frustrated. You are, where we all were, once.
First, congrats on making some garter stitch squares.
Second, you are very brave to go from small squares to mittens. That would likely not be most peopel's first real project, and is probably why you feel so frustrated. While very basic mittens could be doable for some new knitters, getting a good fit requires understanding sizing/fit, needle sizes, yarn substitutions, gauge....
May I suggest that you go back a step...
Try expanding your squares into something larger, but still flat (washcloths, towels, blankets, scarfs).
Try some different stitch patterns (garter, stockinette, ribbing, cables).
Try different size needles (but the same yarn to see how it varies).
Try different weights of yarn.
Try some color techniques (striping, intarsia).
I think once you accomplish this you will have a better undertanding of some knitting fundamentals, not to mention worked out many of the kinks faced by new knitters.
Then you can happily move on to some mittens which, will be no doubt perfect, well-worn and loved.