Welcome to knitting! As others have said, Garter Stitch is very versatile, easy, and useful. I think you will find yourself going back to it even as you learn more.
For me, I definitely have to adjust my work a lot of times when I finish a row. I often find that, when I am knitting something flat, my stitches start to wrap themselves around the needle as I work. So, when I finish a row, I do try to flatten my work out so that it "hangs" from the needle in a neater way. This is hardest to do when you haven't knit very many rows yet (I find that I need at least 3 rows before I can really adjust it properly).
As someone else mentioned, just be carful of how much you are actually "pulling" on it. It doesn't matter so much with Garter Stitch, but as you progress in skill, you'll want to be careful. Some stitches (patterns that make lace, for example) can be damaged if you actually pull on them. However, as has also been mentioned. Much of this is probably to do with your tension as well, which should resolve itself into being comfortable for you as you practice knitting even more. Remember that knitting shouldn't be hard--you shouldn't be pulling the working yarn super tight with each stitch and you shouldn't be forcing the needle into the next loop. Likewise if things are too loose.
You'll get it--keep going!