The most basic bet is placed on the "Pass Line" before the shooter rolls the dice. 

If the shooter rolls 7 or 11, you and the shooter win, and the shooter keeps the dice to roll again. If the shooter hits 2, 3, or 12, the bet is lost but he remains the shooter and continues to roll the dice until he either makes his point and you both win, or he rolls a 7 and the bet is lost.

In the above case, you have bet with the shooter. If you want to bet against him, you place your bet on the "Don't Pass" line and the rules are entirely reversed except that if, on his first roll, the shooter rolls 12, it is a standoff (you neither win nor lose).

After the roll has begun and the shooter has a point, you can make a bet by placing it on the "come" line. The same rules apply to you as if the shooter were making a first roll: it the next roll is 7 you win; if it is a 2, 3, or 12, you lose; if it is any other number, that becomes your "come point" and, for you to win, it must be rolled again before a 7. If a 7 comes first, the bet is lost.

If you are a beginner start with a "pass" line bet and then perhaps try a "come" bet. And don't worry about making an error. The dealers will be happy to explain more about the game to you.