If you're talking about taking odds, there's no house advantage on that,
You are correct. And yes, you must make a pass line (or come line) wager before being able to make this "odds" wager. Some casinos offer "2x odds" while others I have played at offer up to "10x odds." For those unfamiliar, here is how it works:
Find a friendly craps table in a casino.
Bet $1 on the Pass Line. On this particular bet the house has something like a 52/48 advantage over the $1 you wagered.
Throw the dice and "get a number."
Let's suppose you bet $1, threw the dice, and number 4 came up.
The casino lingo is "number 4 is your point."
Bet another $10 and place it slightly behind your $1 bet. This is your odds bet, presuming the house allows up to 10x odds.
The house has no advantage over you on this $10 bet. To win, you must continue rolling the dice until one of two things happens. If you roll a 7, you lose. If you roll a 4, you win.
Here is the important part: it is twice as likely that you will roll a 7 compared to rolling a 4. Without some method to compensate, no idiot would wager that $10 when the odds are against you by 2 - 1. So, in return, the house pays you DOUBLE if you roll a 4 before rolling a 7. This is called true and correct odds. Let's do the money part:
Bet $1 on pass line and roll the dice getting a 4.
Bet another $10 as an "odds bet."
Roll another 4. The $1 you wagered on the pass line wins $1, for $2 total. The $10 you wagered as an "odds bet" wins $20, for $30 total. You risked $11 wagered, and when the 4 comes up you now have $32 (original $10 bet on odds, plus payout of $20 at 2 - 1.)
If you gamble using this technique, you greatly dilute the house percentage against you. It started at about 52/48 against you with the first $1 bet, and you then diluted that a LOT by wagering another $10 behind it where it is as likely for you to win as it is the house.
This, of course, is a very simplified version of craps. But the beauty is you don't need to count cards, or keep track of hardly anything, and it works even after a few drinks. The next hard part is walking away from the table if and when you get ahead and have some winnings.
Now: who knows what the likely worst possible bets are on the craps table. Hint: even someone who doesn't understand the game can quickly figure this out!