Always use the pronoun "tú" when speaking to the waiter. Using "usted" is seen as overly formal in Spain. Also, if you want to address the waiter by his name, use his first name, not his last.
Say "la carta, por favor" if you want the menu. In Spain, "el menú" means combo (in American English). For example, "el menú del día" means "today's combo."
Use the expression "me pones" or "ponme" when you are asking for something. For example, if you are ordering a beer, you can say "me pones una caña" or "ponme una caña" (give me a beer from the tap).
Say "está buena la comida" (the food is good) to tell the waiter that you like the food. Saying "está bien" means that the food was "okay."
Ask for the bill when you want to pay by saying "la cuenta, por favor" (the bill, please) to the waiter. In Spain, it is rude to bring the bill if the customer doesn't ask for it first (though some American restaurants, like Hard Rock, break this cultural norm in Spain).