How well do you know your 'partners'? Are you going to have employees and if so how many? That is an important thing to know and/or to plan for as it has implications on many rules and regulations.
The small business administration has a useful website
Start up . The IRS also has publications about small business taxes etc, check out their website.
Health insurance, business insurance, state tax structure (gross reciepts taxes for example), office supplies, computer services and support etc etc etc. There is much to consider, depending of course, on the size of the business that you intend to start.
-Zeno