When my mother was teaching me to sew, she had me practice on something with stripes, like a old shirt or something. Just to help you run straight lines that were actually straight. You could see if you pulled the fabric it distorted things etc, and helped me keep a gentle grip on things. Practice, practice, practice.
Also, a post it note in a bright colour stuck to where your seam width should be is a lot easier to keep your eye on when sewing than the little line in the plate.
When I was teaching my daughter to sew, she made loads of blankets/pillows and cases for her dolls, or beanie baby 'sleeping bags' and things like that which were all straight seams. I would iron the seams for her if it was something to be folded down like a hem, or draw it on with chalk if she was seaming 2 pieces together.
Cloth napkins are also a good project to practice.