Using your skills and knowledges.

You should think at something you are good at and you can right about it easily.

Then this should be a niche, if it is a general thing ( e.g software ) find a niche ( e.g symbian s60 mobile phones software ) and start making a website. If you are good at design and programming you should build it by your own, if not, use a template or use a blogging software ( e.g wordpess ).
After that apply for a google adsense account, if you don`t have one already.

Then think at a good ad placement, so people can see your ads. The best locations are the header and the main content. So you should insert the ads in your content and a link ad in the header. You have to figure this up depending on your website.

Do a little website optimization, choose some keywords, a description and start making content related to them.

Start promoting your website on forums or other websites ( don`t spam ! )

Submit it in directories ( 10-15 directories daily ) and get to it on related websites.

If you have chosen a good niche and done well with the website, ad placement and optimization, you should see the money increasing.

Good Luck