I've heard that that when taking vitamins, you can't get to much of one. Because what the body doesn't need it excretes. Is this true?

Depends. In general, water soluble vitamins like the vitamin B12 and vitamin c are well tolerated.The fat soluble vitamins like vitamin a, vitamin d, vitamin e, and vitamin k can be toxic if taken in higher doses than the recommended dose.Niacin can cause unpleasant side effects as well as the dose increases.In high doses, vitamin b6 can effect your peripheral nerves in hands and feet(usually reversible ).
Depends. Some vitamins, like vitamin c & folic acid, are water soluble, and any excess of them will just be excreted by the kidneys. Some vitamins, like a & e, are not water soluble but are fat soluable, and too much can be toxic.