I a a 21 tear old ..what can i take that will help me have an erection after i ejaculate?

Nothing: You are supposed to lose your erection after ejaculation. It is called the refractory period. The penis needs to rest. Follow nature's plan and let your body rest a bit.

Not much you can: Readily do. It's your body's natural response to ejaculation/orgasm, and part of something referred to as the refractory period. Besides a prescription medication known as cabergoline(which shouldn't be used for this purpose), understand its something to be expected.