Advertisements are a huge topic. There are volumes of information on this topic. It could be difficult to make heads or tails out of all the advertising networks, and navigate through contextual advertising, impression based advertising, and advertising that you manage in-house.
I am by no means an expert on this topic, but I will tell you what I know, what has worked for us at DSL, and what I hear works for others.
Advertising will end up being your biggest source of income. It is more important to get it right than any other monetizing effort you will do on your site. Don't be afraid of them, you may think that being too aggressive with advertising will hurt your viewership, but it won't. Also, you may want to place small ads in quiet little corners of your page that can be easily ignored, but that is a bad idea. You need to place ads where they are clearly visible. Do not be afraid of them, or what your community will say about them. When looking at your pages the ads need to be in a place where they get attention. Many of the advertisement only pay if they are clicked on, if no one sees the ads, then no one will click on them, and you will not earn any money. Bottom line, place ads where people will see them.