I need to hook up a 220 for a water heater, I think 30amp. Do I hook the white and black wire to each breaker (2 pole)and the ground to the grounding bar... is this correct?

Correct.

In order to size the cable correctley, you need to make certain of the amperage needed.

A 30 amp loads, need #10 AWG conductors.

40 Amp loads, need #8 AWG conductors.

You think, 30 amp? You need to be sure before you do anything.

A typical +/-40 gal storage type water heater is a 240v-30A circuit.

The OP is obviously not sure so to give out advice is complete speculation.