Can someone please tell me, what are the formalities to register a small business firm in india?

i want to start a business firm.. so what are obligations for a small firm to be started in india and to register it. (let it be any sort of business) or a consultancy. please give me brief details on what all i must be careful of??? legal details?? govt. issue??? registration???? a cost estimate for a minimum for small firm to start working... if i have my office n land already there!!!