I would definitly draw up a business plan first.

Who is your target audience? Will you be targeting the lower end of the market or not? Who is your potential customer? Where do these people exist? Online/Offline?

I would research around at the various reseller packages available, don't go with a host because they're cheap! I recommend www.site5.com, www.unitedhosting.co.uk and www.surfspeedy.com for value/performance.

You will need a decent design, invest some money into a quality website design, including integration of some scripts if needed (billing management etc) Don't think the design is the least important, it's probably the most important! You need to create a brand for yourself, an identity and a professional image! Stay clear of templates, hire someone who can do you justice. I would recommend www.4design.tv for a hosting solution.

With your business plan, hosting account and site design you're ready to go! I would start by trying to get a few friends to host with you (perhaps offer them special deal) or try and get one customer (even if you give them hosting for a very low price or free) make them very happy and they will refer others! Don't waste money straight away into banner adverts on large sites/forums, without an established name it won't be worth the risk.

With a solid business plan, decent and reliable reseller account, professional image and smart marketing sense you won't go far wrong!