If you don't know what tile to pick, just pick the next available tile. This will (maybe) get your opponent in front of you on the tile wheel and then reveal new tiles that may be helpful to you.

Don't underestimate the importance of tile-squares with no dots or numbers on them. These come in handy later in the game if you need to play a double XX on them.

In the early rounds, try to pick colours that you need. You start with two *required* colours and you should generally try to finish these first. If you and your opponent(s) need the same colours, it will be extra important to grab these tiles.