LLC can I tax deduct a room in house?

See IRS Pub 587 http://www.irs.gov/pub/irs-pdf/p587.pdf for information on business use of your home. You might want to consider the safe harbor election discussed in the pub. It should reduce the chances of an audit and will alleviate a LOT of headaches if you own your home. (Depreciation recapture at sale time is fugly. The safe harbor option bypasses that.)

You file your return using your home address. As a SMLLC you file Form 1040 and attach Schedule C to figure the net income from the business. Attach Schedule SE to calculate the self-employment tax. An LLC is a disregarded entity at the federal level so you file as a Sole Proprietor unless you opted to be taxed as a corporation.

The first trip out at the beginning of the day and the last trip back at the end of the day are commuting miles and are not deductible. The miles between jobs are deductible. To deduct any vehicle expenses you MUST keep a contemporaneous written log of all miles driven. Without the logs any deduction will be disallowed at audit. You may either use the standard mileage rate for all business miles or the actual cost method apportioned between business miles and personal/commuting miles.

the post office box will not affect your eligibility to claim home in office if the space qualifies

and remember and LLC is a state designation, IRS only recognizes a Corp, a Partnership or a sole proprietor