In Scrabble you get two blank tiles. They can be any letter you want them to. The blank tile has no point value; although it still doubles or triples the value of the word if played on a double or triple word score. From the official tourney rules:
When playing a blank tile, you must state what letter it represents, print that letter on a separate piece of paper, and show this paper to your opponent, all before ending your turn. If you neglect any of these actions, your opponent may neutralize the clock until you comply.

You may repeatedly change a blank tile's designation during your turn; but once your turn has ended, the designation cannot be changed for the rest of the game.