If more than one player has a Flush, then the winner is determined by the player with the highest straight. So, for example, a King-high Flush  in any suit - beats a Queen-high Flush  in any suit, and so forth.

If two flushes have the same high card, the next highest card wins. For example, in a game of Texas Hold'em, let's say one player has K&c T&c, another has Q&c J&c, and the board comes out A&c 9&c 5&c 8&f 8&e. Both players have an ace-high flush, but the player with K&c T&c wins.