You can vote for up to 101 delegates, in 4 batches : 3 batches of 33 delegates and 1 final batch for the last 2 delegates.
You don't have nor need to vote for all of the 101 delegates you can support, but as voting costs you 1 LISK each time, i advise you to vote for 33 delegates per batch.
Voting is possible from your LISK nano wallet.
Voting for delegates allow them to mine LSK : the top 101 voted delegates forge LSK. That's what Delegated POS is about.

Depending on the delegates policies, they will (or will not) "pay" you / reward you from their forged LISKs.
There a some "delegates pools" with special rules/policies, such as Sherwood pool (best one so far : http://robinhood.liskpro.com/voter.html) or GDT pool.