go to https://dpostools.com/LISK/ and enter your Lisk amount there.
Then you can see how much you can earn with each delegate monthly.

Voting is required only one single time. The higher the share rate of a delegate, the higher your earnings are. Unfortunally most of the 101 delegates only share ridicolous 5-25%, so you shold prefer especially the few remaining independent pools which share 40%+.