Obviously, from the outset, you should have a good idea. This should be thrashed out until you're blue in the face. If it's not altogether great then don't go with it. When that's done you have to think of the legal angles. Make sure you have contracts and non-compete agreements drawn up.
Protection is very important when considering things like trademarking.
Not all people who know HTML are necessarily good designers. If your budget is very low, consider getting students involved. There's a lot of kids out there who are only too willing to build up a resume, and these kids today are a heck of a lot more mature and knowledgeable than we were. Finally, create a sound marketing plan and business plan. Consider drawing up a financial model and concentrate on who your target audience will be.