To use an affiliate programme is easy, you only need to add some affiliate code to your existing website and cross your fingers. If you are already getting traffic, many are bound to click your ads - making you money. Once you join an affiliate programme and add the code, the programme itself takes care of the tracking of the clicks from your website.
There are a lot of different affiliate programmes available, and not all work the same way. The main difference lies in the way they calculate and pay commission to the affiliates, and of course in the nature of products they sell.
The most popular methods of paying commission is per sale or per click. There are affiliate programmes paying per lead, but they're limited in numbers.
Deciding what affiliate programme to join can be difficult, but there are many experienced affiliates offering their knowledge in articles, guides and tutorials. On this site I have collected, what I believe is, some of the best articles about affiliate programmes in general, as well as about specific types of programmes; namely PPC and PPS programmes.