Sort:  

There are 21 active witnesses in total. Only one of these is determined through mining, in a way similar to bitcoin.
The other 20 active witnesses are selected through a voting process. Every steem user can vote for witnesses with their SP. I. e. the more SP you have, the more weight has your vote.

So in order to take control of the network, creating lots of witnesses is not enough. You must also get them voted in.

Why should I vote for a witness. Is there benefits or drawbacks for voting

Voting for witnesses helps secure the network.
It also rewards those witnesses monetarily, because it increases their chance to sign a block and receive the witness reward.
There are no drawbacks. In particular, it does not consume voting power. Witness voting is entirely separate from curations.

How do I know who to vote for? heh

Puzzled. New to this concept.