First things first -- get organized! Clear out the room and sort all your stuff into three piles: office, guest room and miscellaneous. Throw out trash and broken items. Toss unused or rarely used things into a donation box. What's in the third pile doesn't go in your office or guest room, so put those things away where they belong or get rid of them altogether. Remember, this is an opportunity to simplify your life and create a space where you actually want to spend time, not trip over unnecessary extras.
Survey the empty room and gauge its potential. Are there items you need to buy? If so, create a list and shopping schedule. Put purchase deadlines on your calendar to help you follow through. Consider acquiring items in stages instead of all at once to make it easier on your budget.