If you're running this as a business, check with your CPA or financial advisor (or a lawyer) whether your state requires you to register the business; could be a corporation, or even a "doing business as". Even if not required, if you expect to earn substantial amount from this, it's worthwhile registering the business to avail of tax breaks for expenses etc.
That's from a business perspective. From a technical perspective, you don't need any of that - signing up for a reseller account is like signing up for normal hosting.
You will need to have either a merchant account or paypal integration if you expect to do transactions / payment online; if your clients are local and pay you through bank transfers / cash / other means, you won't need to set up these.
Quite a number of people offer web design / integration services for setting up a hosting website, and quite cheap too if you use a standard template. Try the web development sub forum here at NP for offers.
Buying reseller hosting - depending on the space and transfer / bandwidth required, you might go with a simple reseller account, or a VPS / dedicated server. You say "collected lots of them" - lots could still be small sites, or huge ones.
As a rough indicator - if your usage for all sites together is less than 100 GB per month, you can make do with a regular reseller account; Between 100 - 500, depends, you may find a VPS better; above that, you're better off with a dedicated server. Note that this is a very rough estimate - you may have specific sites that use a lot of CPU time or other resources, forcing you to consider a different hosting option.
Resellerzoom has some good reseller offers, and I've found their support responsive. Plenty of other offers to be found on webhostingtalk forums.
Hope this helps.