Pass the tiles your best hands need least. You can never pass a Joker, so every Joker you are dealt stays with you.
If the Charleston is new to you, start with what the Charleston is in Mahjong Island.
How do you know which tiles your hands need?
First, look at the Island Card, Mahjong Island's own card of hands. You win by completing one hand on it: 14 tiles that match one line. After the deal, find the 2 or 3 hands your tiles are closest to.
Keep the tiles those hands share. A tile that none of them uses is your easiest pass.
What does that look like in a deal?
Say your tiles sit closest to 2 hands on the card. Most of your tiles fit one hand, the other, or both. Keep the tiles both hands share. One tile fits neither hand, so it is a natural pass.
One habit for your next hand
Before you pass anything, name your 2 or 3 best hands on the card. Then ask one question of each tile: does any of these hands use it? If the answer is no, pass it first.
The same question helps after the Charleston too. A tile none of your hands uses is also your easiest discard.
For more habits like this one, read Mahjong Island strategy for beginners.