This depends on what your goal for the business, where you are going to operate out of, the type of business it is, and other functions of your business.
Use the following to give you some guidance:
http://www.legalzoom.com/business/guide_intro.html
If you want various types of investors (Angel, Venture Capital, etc.) you will need to Incorporate (Symkat Inc. (Mainly just used the United States) or Corp. (Used Globally) though starting out you can form an L.L.C. then upgrade to a corporation once you want to increase the growth of the company.
If you have enough money to bootstrap your project and already have investors knocking on your door you can skip to forming a corporation so you can get funding to help build your product and bring in employees and pay them above market rate so you can get great talent on your roster.
Just be sure to take the time to read up on the tax differences and restrictions on each type of business before forming it. If your doing a corporation you will normally be advised to incorporate in Delaware or Nevada if your in the United States.