Well for an adsense site you must relaise firstly that their are no promises - it your site is popular you will definately make money, if its not you will make very very little. Secondly a knowledge of how to create websites is needed. You should understand html and how to generate/create/write it.
If you do set up a website the most important thing is to do something usful, people will only visit if your website serves a purpose or fills a gap in the market - DO NOT CREATE A WEBSITE WITH THE SOLE AIM OF MAKING MONEY. My first website never got anywhere, my second proper website however is looking very promising, it is only 2 months old, earnings are literally under $1 a day. However I am certain this could rise to well over $3-5 a day in a few months. My view is that it would become a passive source of income; maybe an hours maintence a week but still generate maybe $1,000 a year. Obviously the idea would be to grow this in the long run by creating new websites. I cannot tell you my exact earnings because this is against Googles TOS.
Adsense is not for everyone, it is more appropriate to cash-poor, time-free teenagers who can't make much money any where else!