what determines the ownership percentage?
Generally, it's assumed to be 50/50 unless specified otherwise in a legally binding document of some sort. But I stress, "generally".
If not specified in writing, other methods include things like what percentage of the down payment each provided by each owner, what percentage of each payment was made by each owner, etc. Things can tend to get more complicated when your state also taxes personal income.
Overall, I would suggest you both obtain the advice of not only an RE professional, but a tax professional in your local area. Especially if the intent is to rent out the property. You may find it best to form a partnership or multi-member LLC in that case. Doing so allows one to specify the "capital contributions" of each member to the partnership/LLC, as well as specify ownership percentage. It can get complicated when you get into things like general partners and limited partners. I've never got that deep into partnerships or multi-member LLC's myself, so can't be of much help in the more complicated situations.
Overall, with a partnership or multi-member LLC, the "business" will file it's own 1065 tax return and issue each member a K-1, which each member will need to complete their personal 1040 tax return. Turbotax has a separate program called "TurboTax Business" for that. (Not the same as Home & Business, which does not "do" 1065 returns.)
