Hey,
Firstly, welcome to HostingRefuge
The first thing to consider about starting a hosting company is if you have the time to support your clients and enough knowledge to allow you to run a business.
If you want to start small, you can start off with a reseller account as others said.
A reseller account allows you to setup accounts for other people which you are the "owner" of.
With a reseller account you dont have to worry about server admin as someone else will take care of that for you (the server owner)
There are a lot of limitations with reseller accounts including that you cant install anything on the server, change the main configuration of the server, you must depend on your upstream host to help you with problems on the server side etc.
The next option would be a VDS/VPS.
Thats like a dedicated server only its a dedicated split up into different sections, and you will have one of the sections. That gives you root access so you have a lot more control over the server, but still a few limitations set by your host.
You get less space/bandwidth per $ that you spend but its not too expensive, and normally there will be someone else managing your server also.
Then the final step would be a dedicated server, which costs the most but gives you complete control over everything on your server.
They cost from roughly $99 up, depending on what you want.
Then you have to buy your scripts, add a price for a control panel monthly, get a CC processor/merchant account which could be monthly fee and take money out of transactions, or just take money from transactions.
You might need other support personnel but normally you wouldnt at first.
You might also need to hire a server admin if you dont know how to admin and secure your server, which could cost a good bit if you only have one server.
If you wanted to advertise once you got started the price varies by a huge amount, depending on where you want to advertise, when, and how often.
There is a lot to factor in so its something you really have to think about and do a bit of research into before diving on board and realising its a lot more work than you expected.
Hang about here for a while and ask as many questions as you have and try get a bit of research done, then when you think your ready, start asking for help on which company to go with, what scripts to get for your company etc. etc. and you will soon be on your way.
BUT, as I said, its a lot of work and work out if you have time to support your clients, it is a business and people will be depending on you, sometimes even to keep their business online.