Help with newbi crochet?

I have been praticing different stitches- only single and double. I just learned BUT my problem is the stitching looks great if i could cut all the edges off my scarf...lol.. why are all my ends either curving (like a rainbow shaped scarf ipo a rectangle), or its much wider on the ends - slanting down and tighter and smaller as I go?am i messing up in my turns when i restart a line? Also im using a G hook is that ok? Thank u sooooo much.