This link may help a bit http://www.webhostingtalk.com/wiki/Colocation
I have also learned a ton about colocation simply by browsing the colocation section http://www.webhostingtalk.com/forumdisplay.php?f=44
Normally, you will have to buy your own servers but there are company's that you can get servers via lease to own methods. Which is generally paying for the server monthly for a certain amount of time until the server is paid off. At that point you own it.
With colocation a few things you are going to have to deal with is paying for space, power (Where I am it's about $18 per amp and each high end server is consuming about 1.5 amps) That power costs to fill a rack which is about 30 servers is $800 alone in power costs. Then there is bandwidth which pricing will vary from a dollar per mbit to 5+ dollars per mbit. Then there is the switching and routing equipment to go with it. For me, this wouldn't be feasible to start. It would save money long term if I were to start a business and do this but I wouldn't be able to start without getting some serious equipment.
Then you also need PDU's, if your facility isn't giving you a bandwidth blend already you may have to set your own up for redundancy sake between multiple bandwidth providers, the costs to do this long term isn't cheap. All of the cabling for a full rack. Sometimes $400+ just in wiring. Spare hard drives, spare power supplys, spare ram, on rare occasion spare cpu's and chassis and so on. You also need to know how to manage all of the equipment. Some if it is pretty easy and some networking related tasks can be extremely overweighting for some and time consuming. It would probably be easy to take on a small chunk of information at a time and paying for courses online or at schools before starting over a course of months or even longer.
When you first start out it is usually cheaper and easier to just rent dedicated servers from providers and let them do all of that. When you have 10+ servers then it starts becoming more feasible to do so. You are going to get major bandwidth discounts when you buy bulk amounts. You won't make much of a profit depending on who you go with just paying for some bandwidth at an insanely high price for a server or two. Then if a client of yours gets a DDOS attack, you won't usually have the bandwidth capacity to handle it. Everything could possibly go offline.