I'm not really good at describing stuff but I'll do my best.
To make a straight line you need to do it in a sort of back and forth motion, so you can control where the line is going. Also don't apply too much pressure when drawing lines, because then it's going to be pretty hard to fully erase the line if you make a mistake. I recommend white erasers over regular pink ones. The pink ones don't get rid of graphite as good as the white erasers.
Also don't erase something that doesn't quite look right at first. Try to build on it to make the shape you want. Then you can get rid of the stuff you didn't want initially. Take it slow to make detail.
