You can make money on a porn site using affiliates or creating your own content and charging for membership. That seems to be the primary two ways to do it. There are many programs out there that help you get set up, host your website, and even give you content. You just need to promote their affiliates and they take about 50% of the cut. It may be worth trying that first to see if you are really interested.
Somebody was talking about Adbrite being used a lot. BlackLabelAds is run by Adbrite and is made for such sites.
Porn sites can be very profitable, but you have to find the hot micro-niches. For someone who is brand new to porn sites, there is no way you will be able to make any money going after the typical stuff like girl-on-girl or big jugs (I'm trying to be as pg-13 as possible). Instead, if you want a real chance to make any money in porn, you will have to go after a micro-niche like "interracial girl-on-girl with big jugs". I just made that up as I wrote it, but I'm almost sure you will find a few sites dedicated to that micro-niche already. Another thing to keep in mind is the impact that Porn Tube Sites are having on the industry. People get all the porn they want for free by visiting the tube sites. Unless you have a unique selling proposition that will convince people to spend money on the porn you promote instead of watching it for free on tube sites, you may have trouble making money. I run about 60 or so porn blogs. My conversion rate used to be about 1:1000 (which is pretty good in the porn business), but in the last 6 months, my conversion rate has dropped to about 1:1400. Still not bad, but for people who try to cross over to the porn industry from mainstream marketing, the conversion rates probably look crappy. The reason I shared my conversion rates with you is because I want you to know what you are getting yourself into before building sites. If you suck at SEO, don't have solid traffic generation skills, or hate getting 1 sale out of 1400 people who click on your offer, stay away from the porn business! If you have solid SEO skills, you understand how to drive traffic to websites, and you can handle getting 1 sale (usually $30-100 per sale depending on the sponsor and program you choose) out of 1400 people that click your offer, go for it! Recurring commissions are usually good too- but you have to find a sponsor and program that is good at retaining their customers.