Beginners Crochet Question?

I just started to crochet and I need a little help. I want to start off by making a few little square pot holders or maybe a simple long scarf. The issue I am having is making my edges strait. They are beginning to curve. Do I need to make a double crochet at the end to prevent this? Can anyone explain to me what I need to do in crochet terms so then I can look it up online? Or if you have a website or video you can recommend that would also be great. Thanks so much!