Answer:
Level up earning gourmet points! You earn gourmet points by just letting your employees serve their costumers, but if you want more, you can visit and help out friends in the game ;) There are hacks too, but they kind of ruin the spirit of the game... Some levels expands your restaurant, some gives you a new employee or two, and every level gives you coins.
-->But recently, Playfish added a new feature which made it possible to buy a little extra place for 5000 coins, as an outside area. The only sucky thing about that is that you can't lay floors or walls on it. But you can still put tables
You can only make your restaurant bigger by gaining ranks. When you gain ranks in the game, you usually get a surprise and a larger restaurant. You can gain ranks by keeping your restaurant clean, updating it, visiting friends and etc.