To start a new game, click with the right mouse button in a game window and select New Game from the popup menu (or use the New Game icon in the status bar). The game number is randomly selected from the first 10 million of the 2 billion possible deals.
The game card piles will be cleared, new deck(s) added, shuffled, and redealt. If a game is in progress and has progressed beyond a few moves, you will be prompted to verify a new deal (unless you have checked Skip All Confirms).
Shortcut: Use Ctrl+N (the Ctrl key and the N key pressed simultaneously) to start a new game.