I've had many affiliates ask this question over the years. I would definitely follow the advice of all those posting here.
You'll most likely need to learn html, design, marketing and all that, but first, you need to figure out what you are going to talk about. And how do you do that?
Something I always tell a beginner is to start with what they know best. Start where you are passionate. I personally am passionate about cycling and my kids, so if I were to start an affiliate site, I would probably start in those two areas. Since I am pretty knowledgeable in cycling and triathlons (I race competitively) and I have been a parent for 10 years, it would be much easier for me to develop an intelligent site/web property about these two issues.
It enables you to more easily demonstrate expertise and/or authority in this field without seeming contrived or overtly "sales-y". Not that that is always a bad thing
Once you have determined your niche, then you can brainstorm ideas on how to turn that into a moneymaker which will lead you to questions like: "what do I build?", "how do I build it?", "how do I get traffic?", "how do I make money at it?".
Hope that helps!