How to Join AdSense

Once you have 10-15 pages on your site, go to http://adsense.google.com to apply.
When you are accepted, simply copy and paste the provided HTML code into any page that you'd like to show the ads. If you've done a good job of defining the content on your web pages, the ads that show should be relevant to the content of your page...increasing the chances of click-thrus by your visitors.
You can either display the ads vertically along side the page like Google does or in a banner-like formation horizontally across your pages.  The placement is up to you.  You can even customize the colors to match your site's theme.

Tips on Succeeding With AdSense

Here are some tips for achieving success with Google AdSense.
1.  Create a website with your (YourSite.com).  Don't try to use a free web host because your site will likely have banners and pop-ups and get rejected because it looks unprofessional. Not to mention, a free web host will give you a website address like this:

http://www.thefreewebhost.com/yoursite/member1234/home.html

instead of...

http://www.yoursite.com

Start by registering your own domain and then select a web host.
2. If you don't know web programming or have no desire to learn it, get a beginner's design editor like Dreamweaver (what I use) or CoffeeCup.  You can publish your content directly to the web from the software.
Keep in mind you can't use the software alone.  You must have a web host that will allow you to publish your site to the Internet.
3.  If the main goal of your site is to make money with AdSense, be sure to choose a topic that you know a lot about so you can write lots and lots of content.
4.  Get traffic. Once your site is up and running you'll need to learn how to get your site listed in the major search engines. 

Getting into Google is completely free and can bring in hundreds or even thousands of visitors per day.  All you have to do is submit your site to them and wait patiently while Google adds it to the index.  It can take a few months.  Patience is key with Google.  Then do the same with Yahoo, MSN, etc.
Read up on how the search engines rank pages here.

And while you're waiting it's important you build up as much unique content around your theme as possible. Google loves large sites with useful content centered around a specific theme.

Don't create a hodgepodge site with topics on everything under the sun. Stay focused and make sure your site has an obvious theme.