Some ways to do this are to sign up as an affiliate for companies and post there banners on your website or blog. The best places to do this if you are using a blog are blogger and wordpress. Also if you start making money from it and want a domain they make it easy to set up.
Then you can just add the banner to your sidebar and anyone who clicks on it and makes purchases will go towards a percentage for you.
Another thing you can do is to sing up to receive free products for review. Companies will send you a free product and then you just write a review about there product or company and post links to it in your blog.
It might take a while to start making money from these things. Another way to make money from your blog or website is to create e-books. If you know a lot about something then you can write an e-book on the subject and offer it on your website or blog for a minimal fee. You don't have to pay tons of money to self publish it or go through all the hassle of having a publishing company pick it up.
You can release an e-book on a number of sites. I have put some on lulu.com which is a self publishing site. You can also post anything on there for downloading like pictures or music for example.
blurb.com also has self publishing and you can publish e-books on there as well.
