Brainstorm a list of every thing you will need to buy for the business to get it running. Also add any items you might want to purchase within the first year if things are going well. Then research the costs of your overhead including utilities, insurance, rent, etc. Your capitalization should be equal to all your purchases and expenses for the first six months.
Your start up costs will vary significantly with the type of business you are starting. Some examples from my clients include:
Type of Business / First Six Months Expenses
As varied as these start up costs are, the point at which different companies earn back their initial investment is usually 12 months. Surprisingly the sixth month seems to be when start up companies go into the black for the first time.
Also it is a good idea to generate some revenue before you start buying stuff. Technically start up costs need to be amortized instead of expensed. By billing someone for some nominal amount of money you can then deduct the expenses from your taxes. The exception would be vehicles and equipment which need to be depreciated. Of course the current section 179 deductions are generous. Consult your accountant for details.