A partnership is a partnership, even if it is not registered officially with a government body. You need to file a form 1065 partnership return. The due date is March 15, not April 15, and the penalty for late filing is $195 per month per partner.
To use Turbotax, you would need to use the Business version which is only available as a download or CD for PC, there is no online or Mac version. When you prepare the 1065 you will create a K-1 statement for each partner that is added to your personal tax return along with any other income and deductions, that you can file with one of the personal versions of Turbotax.
However, it would be wise to see a professional for the first year.
My recollection of startup costs is that if less than $5000 you can expense them, if more than $5000 you can expense part and amortize part, according to a formula that I don't have handy. And any of your startup costs that are assets (equipment, etc.) is depreciated in one of the normal ways rather than being counted as part of the startup costs. But again, see an expert for your first year.
