The Tower is on an island south of Route 130. However, you cannot Surf there; you need to
take the ferry from either Slateport or Lilycove.

should i beat eliet 4 first?

Yes.

Step 1 Beat the Elite 4 Talk to Norman and he gives you a ticket go to slaport or lilycove
then go the dock. Talk to the girl and there you have it battle tower.

first beat the elite four, then continue the game, you'll be at home after the long
worthless credits, and your dad will talk to you and he will give you a tiket, then go to
either slateport or lilycove and take the ferry to BT

beat the pokemon leage and then go to a city that has a boat stop and talk to the person
by the boat