If you're going to start a business, it better be a business, not just a tax dodge. That means a business license from the state and city, getting a fed tzx ID number, and keeping some sort of books. It also means showing that there was actual business activity going pn. If you do this as a lark to get write-offs, the IRS is going to eat your ass for lunch.
Even if this is the case, be very careful about how much depreciation you claim for a computer that is used 90% for personal stuff. Same for "home offices". These are really sweet items in an audit. I've actually seen IRS guys drool all over their shoes.
As for what I deduct, obviously all interest on my home comes off the top. State income tax withholdings (less any refunds from last year) come off. Property taxes (including asd velorum on car tags) come off. Charitable contributions (like books to the library) come off. 401k contributions and 125 cafeteria plan deferrals don't even appear in Box 1 of the W2, so that's automatic.