Good topic, but awfully broad to approach in a single post. I think there are similar tactics that crossover to any means of marketing - for example selling a product, diverting through an affiliate link, or presenting adsense ads can all be approaches to generating revenue online. If you are just getting started, and need to get up to speed with seo/sem - then I'd start with jen's ebook (free 30 days - http://www.searchengineguide.com/ebooks/freeebook.html
and small business guide to search engine marketing
). Just setting up experimental sites and building traffic through seo - then in turn generating revenue via adsense is an approach many take. This always works (generates most revenue) for me. Providing ads that compliment the content and search queries related. The closer the better.
Outside of general approaches to targeting traffic, you then get into using channels and tracking to a/b test various layouts, etc. If you are new to this, you can read up on things along with checking out the google adsense heat map (https://www.google.com/adsense/suppo...&answer=17954)
. Positioning plays a big role, along with blending the ads to look similar to your site's other links/backgrounds/etc - as much as possible.
With traffic and placement in place, I think then it is important to continue to expand your content to good keywords/markets. I'll often run an adwords campaign side-by-side to test/verify search popularity. Keyword research, knowing how to get first page above the fold rankings, and then last but not least vary up the ads and placement some. Be selective where you do use ads - not every page needs them in the same place, research your traffic - and if a particular page that is drawing traffic that is complimentary for adsense ads then place ads that will generate revenue. If ads are in place, and don't draw revenue (check the channels setup) then don't keep them there in my opinion, leads to ad blindness. I usually don't setup ads until I am drawing traffic to a page - as i think it can be a hinder to have ads in place when promoting a site - especially if there are many/and they aren't drawing revenue.
Find a topic you enjoy, consider focusing on provide quality informational site(s)... the more you work on it, the more (most) will earn.