A couple things about Google before you start though.

1. Read ALL their terms of service
2. Now do it again.
3. You must have a website up and running that complies with their terms of service.
4. Under construction websites won't work.
5. NEVER click on your own ads. or use any Google produced tool that's yours.
6. Don't get your friends to click on them either, or even friends in other states, the Google will know!
7. Don't purchase traffic to pages with Google Ads on them.
8. Adsense is Free, Adwords is what costs.

Google requires that you have a website up and running to be eligible for approval.  Also, the bottom line, the more unique and content rich your site is the better.  The more centered around a particular niche or theme the better.  Basically anything your site can do that focuses are targeted set of users the better, you'll make more money with Google than if you didn't have a site like this.

Google is very fickle and has no problem suspending your account if you mess up.  They are not in short supply of webmasters doing it correctly so if you loose your account it's hard to get back, and most likely won't.  If you have lost your Google account Search Feeds is another great Pay Per Click program you can join instead of Google.

CB Pro Ads is yet another option, but instead shows Click Bank ads in the form of Google Ads, and each ad is embedded with your unique Click Bank ID.  This is a nice feature since most Click Bank products pays you at least several dollars, in a lot of cases more than $40 per sale.  Not bad if you do this passively a couple times a day.

When you first start making money with Google it will most likely find yourself making pennies.  Look at this like Gold.  If you were digging for Gold and found a few specks would you stop?  Or would you get excited and dig for more?  This is how you should see your pennies, as Gold specks and that means you are on the right path and need to keep digging for more.  Don't get discouraged with Google Adsense, and in all reality don't plan to make a living off of it.  Although it is entirely possible and webmaster do it, the truth is this is best viewed as gravy money, since it can be gained so passively. 

So be sure to read all the important stuff with Google, they are one of the few you actually want to read all the terms of service.  It DOES matter.