I'm 14 and play soccer on a team. I am small and not very fast. I'm not fully physically developed yet so I don't know if certain exercises could stunt my growth or cause injuries, etc. Does anyone have any advice?
A standard go-to for improving speed is wind sprints. Basically, you repeatedly go from 0 to as fast as you can and sustain it for a short distance, recover for a few minutes, then repeat. It develops explosive power and, if you have an external starting signal like a friend with a whistle, it helps you develop reflexes so that you start moving as soon as you think of it.
And don't worry too much about damaging your growth. That's only really if you're in the upper echelons of effort, such as professional gymnasts, or where you're being forced into repeated impacts in a way that encourages you not to fall properly, such as American football.
To complement your sprinting routines, I advise to start doing squats and deadlifts. Read around to know how to do them properly but once you start getting the correct technique, you can start adding the weight so that your legs become stronger and will greatly benefit your soccer experience...it has for me!
Tips: I usually do 2-3 warmup sets starting at 135 increasing the weight to 225 for my first real set. Then I usually try to do reps of 8 x 8 x 6 x 4 where each set is pretty tough to complete by adding weight. Next time you redo squats or deadlifts if the last set of 4 was pretty easy and you feel you could have done 6 for example, start the 4 sets with 5-10 more pounds.