There are really, two main ways for you to start selling webspace:
Reseller Web Hosting:
This is where you order a hosting account from a 3rd-party company and they run the account, but keep your anonymity. This allows it to appear to the client as if you are actually selling the space when, in actual fact, the parent company is doing so - you are sort of the "middle-man".
The parent company is only there, however, to provide the hosting, most times they will not provide support and will not contact your clients in any way.
Examples:
www.hostgator.com
www.site5.com
www.donhost.co.uk
www.sonataweb.net
Dedicated Servers:
This is really only used if you are planning on starting a fully-fledged web hosting company! This is where you rent the servers for a set amount per month (usually between $200 and $600 per month) and you "own" the server.
This server is then yours to do with as you will. The hosting company you are renting the server from will either manage the server for you or leave it up to you (often-times, managed servers cost more). It is also up to you to provide support for your clients as well.
Dedicated Servers cost more, but it allows you to, within reason, completely custom-build your own server to what you need.
Examples:
www.allmanaged.com
www.theplanet.com
www.servermatrix.com
www.ev1servers.net
Well, hope I helped, just explained a bit more on the two main methods of selling web space.
Will.