Well, the first thing to do is select a control panel for your shared hosting clients. The control panel would give clients limited access to his/her individual account settings, while still allowing you to completely control the server. cPanel is the only well-known linux panel (Plesk is only for Windows servers), but there are open source panels out there as well. However, cPanel is probably the best one to use as it's the most functional and comes with a ton of community support.
You would then want to build a site that would sell your shared hosting. WHMCS is an excellent client management/billing web app that has plugins for all the major control panels and payment gateways. However, like cPanel, WHMCS is commercially licensed. Unlike cPanel, I'm not aware of any free alternatives that are able to interface with cPanel.
After that, its a matter of marketing. Make yourself known on forums like Webhostingtalk, put yourself out on Webhostdir, and advertising on AdWords doesn't hurt.
I'd be careful, though... It's very easy to oversell yourself and then not have enough server resources to go around. It's also easy to encounter the abusive client that runs a process hogging all the ram on the server, causing it to crash. It's also expensive to increase your total RAM capacity.
Essentially, managing a web hosting business successfully is a full time job. However, if you still want to pursue it, have fun.