You need to buy what's known as a "Reseller hosting account". It's just like a normal hosting account in that you're given so much disk space, bandwidth, email accounts etc...however you have the ability to create your own packages from "WHM" (Web Host Manager, a tool you'll find within cPanel)...you'll also be able to manually create, delete, monitor, suspend hosting accounts etc...
So, for a post to Host forum, a simple way of doing it would be to buy your reseller, create a few different packages eg: Package 1 = 10 posts a month, Package 2 = 25 posts a month, Page 3 = 50 posts a month. Then you could create an actual forum whereby people are required to post for their hosting. Within this forum could be a "Request" forum, whereby people request their hosting and you PM them their account information. Alternatively you could set up a simple contact form and allow users to order from there. Finally, it's down to you to check each month if the user has filled their required post count.
Unfortunately if you didn't understand any of the terminology in my above post there's no point in even trying. You also need a basic knowledge of cPanel and its features as you'll naturally be required to offer support to users who need it. WHM is fairly easy to understand.
It's also worth mentioning that it can be quite expensive to run a "good" Free host. An online associate of mine run one and soon found the running costs heavily outweighed the income. The only income you're likely to make on a post to host site is through Ads, sponsorships or paid upgrades. It's not really worth setting up paid upgrades until you're at least a few months down the line.
Finally, bare in mind the competition you're facing. Although they're particularly good, x10hosting and 000webhost are two of the most popular free hosts, with the likes of fi.st, Ceege and other post to host companies growing at a decent rate.
If you're serious about doing it, do it properly. Sit down and write a plan of where you want to go, what you want to achieve and write down some monthly targets. Calculate the forecasted running costs and also think about how much money you're likely to make, and how you'll actually make it. Finally, be committed. There's nothing that will annoy your members more then if you randomly shut your service offline one day and cause them to lose all of their website files (remember, some may not have taken backups). You may even be liable to legal action if you go offline without notifying your customers.
With all that in mind, I wish you the best of luck in your decision. 😉