What causes my MCV to be low and RDW to be high? And how to prevent it?

Iron supplements. Low MCV points to iron deficiency. High RDW points to the body making blood cells at a higher rate than normal. An iron supplement might be a good idea, be it from a pill, diet or both. However, you need to check with the doc who ordered these tests to make sure there isn't something else going on.
Get workup. You may have that minor which is trivial, iron deficiency which is very serious, or something else.