You can join a fleet by tapping the "Fleet" button that you can find on the main screen, next to the "Battle" button.
In the "Fleet" Menu you can search for and join a fleet.
Please note, at this time you cannot invite players to your fleet, the players have to request an invite.
You can create a fleet by tapping on "Create Fleet". That will cost you Silver.
Please note that, as a commander, you will be automatically demoted from that role if do not log in the game for 30 days.