There are entire books written on how to become a virtual assistant, so this job is definitely worthy of more research. My advice: Focus on one niche (I just hired Lisa Morosky of VA for Bloggers, for instance.) Also, reach out to potential customers directly and let them know what you can do for them instead of hitting up freelance web sites. Finally, you may want to consider resources such as AssistU, which provides training for up-and-coming virtual assistants.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.