Hi Jennysbelly, what an exciting change of life for you! It's definitely going to take some work and your ideal home office will change over time, but I'd advise for your essential first step, taking EVERYTHING except the furniture out of the room, putting it somewhere else, and "interviewing" it before it goes back in.
You'll want to be sure that ONLY what you can *and will* use for your studies and TA duties stays in that room. It should be peaceful, comfortable, and efficient - not distracting. So give your old things a good talking-to and figure out if they will be of use to you going forward, or not.
If you have things that you know you will not have time for while in grad school, now is the time to organize them and pack them away for later - or give them away, if you think you won't return to that activity, or that your tastes may change to the point that you would pursue it quite differently. Remember, it's okay to say goodbye to old hobbies (see thread on unfinished projects).
As you get deeper into your studies you are going to want to have space to accommodate library books, study guides, piles of homework to be graded, binders or folders of class notes, etc. You may or may not want a file cabinet - probably a standalone rolling cabinet that could fit under the desk, possibly with a printer on top, would be most efficient. If the desk is not already computer-friendly, see about installing a keyboard tray and make sure the desk is positioned well to keep glare off the screen and to make power supply easy.
If you don't need the closet for clothes or other storage, you might consider setting that up as your records and materials center, so that all that material isn't out in the middle of your space. Inexpensive shelves or cubbies can be arranged to fit the space and you can close the door on it all when you choose to.
And if you have space to have a comfortable reading chair with a floor lamp (or better yet a lamp hung from the ceiling), that can be a great way to stay in the space with all your tools and yet not be hunched over your keyboard. Visit thrift shops to find a decent old recliner and cover it with a pretty throw.
Good luck!