First of all, what is contextual advertising? Basically, this is a kind of advertising where the main website you sign up to scans your blog with a bot for keywords. If preferred keywords are found, the main website then highlights those keywords and links them to the advertiser's main page. On the other hand, you can also advertise through their website and type in your preferred keyword. That way, people who sign up and enter that keyword to their blog content, the keyword may be linked to your blog! But don't think that there are no competitors on the same keyword. Always choose your keyword wisely to reduce competition but with a sum of bloggers who may type that keyword.
Alright, now you understand the concept in contextual advertising, here are some links to let you join in contextual advertising today!
As far as I know, TribalFusion has very strict requirements that you must achieve in order to participate in their program. Think you're capable? Check it out then! Personally, I recommend Kontera as i use it.