Clickbank is a great place to start as an affiliate marketer. Choose your niche and product you want to promote. Scan through the Clickbank marketplace for ideas. It can be easier if you choose something you have an interest in yourself, but it is not essential.
Do some keyword research, and find keywords in your niche that have a relatively high number of searches, but relatively low competition.
Set up a simple website reviewing your chosen product, and add an optin form to collect subscribers you are interested in your niche. Give people on your list lots of useful information, build a relationship with them, and occassionally promote products that you think they would find useful.
Keep adding content to your site, optimized for your chosen keywords, and drive traffic to it through article marketing, forum posting, social marketing, etc.
Just do it step by step, and you'll soon find that these sites are very simple to set up - and can be very profitable.