It's all a crapshoot. If you are not willing to put up your own money, the tournaments that offer big prizes are going to have thousands of entries.
One of the ones they advertise right now is playing poker at Atlantis with Daniel Negreanu. That prize package is worth well over $12,000, with the potential to win a million. But you'd need to win three or four tournaments in a row to get this prize.
Then there are sites like NLOP.com which offer free tournaments with tiny cash prizes. These are easier to win.
So if you will not actually put up real money (which is a pretty silly way to try and play poker, which is a gambling game), then you can play for a reasonable chance to win a tiny amount of money. Or you can play for an incredibly unlikely shot at winning a large amount of money.
You should read the article by Jesus Ferguson about going from $0 to $10,000 on Full Tilt Poker.