Sign up for affiliate programs with ecommerce sites where you get paid a commission for each sale you make. Then, register for an account on Google AdWords and promote the product you're selling. Do it well enough and the amount you invest with Google AdWords will be less than the commissions you'll earn.
As Dani said, join affiliate programs like Google Adwords, Amazon, Adbrite among others. You must open an account with them and submit all the pertinent information. Then sit and wait for approval. Once approved, just take the affiliate link they will provide you and place it somewhere visible on your site so that visitors can click and you start to earn money.
Joining those larges networks can be profitable, but the best way to make money if you're willing to put in the effort is to join a smaller network. Adwords, Adbrite, Adsense, all those networks are fine, but when you're in them, you're just a number, you've got little room to go up, except by increasing your volume. This, combined with constantly decreasing profits per click because they skim a large amount of the profit from you, is why you don't really want to join a large network.
I've been doing work with a decently sized cpa network called CPA Dough for a while now, and so far it's made me a ton more money than any of my work with AdSense or AdWords. Plus their staff is very professional and helpful, when I was starting out, they sent a lot of offers my way, and the amount of offers they get is simply absurd, so if you have some time to invest, I would definitely check it out.