Think of affiliate marketing as way of selling product of someone else. For example, you've to sell book or software of some person and when you do that you'll get some commission. There are many such sellers on clickbank, linkshare, revresponse site. You can sign up on those sites and then get a code for the product that you want to promote. If you manage to make sales then you'll get commission for that product and you'll be paid if minimum payout is reached.
So my suggestion is to register on either clickbank, linkshare. Collect the affiliate code for the product you would like to promote. Start writing hubs and use maximum 2 affiliate links in one hub.
Have fun promoting products. Keep writing.