You will know very quickly if you are getting a return on investment. If you are buying traffic from multiple avenues, be sure to key the landing page. I hope you know PHP/MySql.
If you do not know how to do this, stop, get a "For Dummies" book, and learn. Otherwise, simply give me the money and I will waste it promptly for you. You will lose money on some sources and, if you are lucky, you will make money on others. In Month 2, STOP buying traffic from the sources that lose money for you and try others. It's really that simple.
There are literally thousands of sources of traffic, and some of them are the same traffic being marketed or marked up and re-marketed under different names. If you are not analyzing the individual sources, again, just go ahead and give me the money.
Another fun thing about bought traffic vs Social or SEO: When you run out of money, the traffic will STOP. Oh, sure, you might get a teeny, tiny loyal following who like your logo colors or somehow think your site is the only spot they can find that video of the prettiest porn star they've ever seen pinching her nipples, but as CyberHustler mentioned, everybody and their pet iguana has a tube site.
So let's back up - You asked your questions in reverse importance. If you are not clear on how to best monetize your site, why do you need any traffic at all? Make sure you have some way to get money FROM your visitors before you spend money ATTRACTING them.
If you're an avid GFY reader, you'll see that a great deal of bandwidth is spent on rants against slimy affiliate programs. For now (and for some sites, maybe forever), a better bet for you than trying to weed through the - again - THOUSANDS of affiliate programs out there to find a good, reliable fit for your site is to simply run banners from an ad network that pays per click. There are a few good ones, and I have never had or heard of a bad experience with JuicyAds:
http://www.juicyads.com/ref2.php?ref=4879
Dating sites and Webcams are also often profitable for tube sites, and you can find many recommendations for those throughout GFY.
So, have fun! And have plenty of Kleenex, because we all get our noses bloodied up from time to time, sometimes from each other