Hopefully I read the OP correct and I will respond how I read it. It's early so give me some slack.
Everyone starts somewhere and if you have a decent reseller plan - then you don't need to be concerned with getting a dedicated box so soon. Learn the reseller plan inside and out to get a very basic idea on what is to be expected.
Also - if you already have a reseller plan, then you are currently using a control panel that you can offer your clients but in case your still interested, I would recommend cPanel or HShpere.
On the billing script - since your starting out - I would go with WHMCS because it has all the features you will need and will help cut down on the cost of a helpdesk since there is a tight one already included that offers KB, Tickets,, Downloads, etc. (www.WHMCS.com
If I were going to jump into the arena of hosting with no to limited knowledge of how it all works, I would move very slowly. Create some fake accounts and learn how to manage and set those up. Try offering your hosting to a few friends that would be more understanding if something goes wrong. Offer them a reduced price to compensate for your learning. You could even ask them to make some trouble for you so you can learn by fire what steps you need to take to fix issues.
Best of luck to you and who knows, you might find yourself years from now being the next big thing.