First and foremost, you need LOTS of money. If you want a real data center, there's a lot of planning involved. And to build a data center that's not a complete joke you're going to need at least a few million dollars. You will also need to hire an engineer so you do things the right way.

There's all kinds of things to consider. Power requirements (UPS, Generator, wiring), cooling (HVAC, raised floor, etc), bandwidth availability, seismic bracing, security systems and secure entry, full redundancy, etc.

Of course you can always stick an air conditioner and some telco racks in a closet and call that a data center, as many people do, but if you want any serious clientele that won't fly.

Bottom line, opening a data center is serious business and it takes a lot of money and planning. If you don't have a few million to burn and a good idea of what it takes, you won't get very far.

Regards,
Aaron