If you have "no use" for it, definately sell it. If you decide to sell it, consider a few options.
1) You could go to a local FFL (federal firearms licensee) at your local gun store. However, that would NOT maximize the value you are looking for. Unfortunately, while I try to support my local shops, they usually don't have the customer base that would maximize your sale price. Sure, you might find a collector who knows that they are all about and might give you a fair price for it, but you really need to broaden your exposre.
2) You should have someone who is very knowledgable about military rifles (thousands of here at this forum) provide you an honest appraisal of that rifle. They will point out the deficiencies and assest it has. You should pay for that written appraisal, too. You're taking advange of the collector's knowledge. A fair collector, knowing that you are not selling it to them, will set a price range that the market dictates for these rifles today. As the other poster said, these are very rare and super desirable. It will generate a LOT of interst. Trust me.
3) Then, after knowing what it's all about, you could simply sell it here at the FOR SALE board. Take lots of good photos, put together an honest description (based on the appraisal), and simply set a price that you are comfortable with. Since you are not experienced with miliary rifles, having someone who is look at it for you is critical.
4) Or, as an alternative, you can put it up for auction. There are a few good sites for that. There is one that is followed pretty religiously here:
http://www.gunrunnerauctions.com/index.cfm
They've recently handled other Johnsons and they have always produced fantastic prices. There are other auction sites, too, (AuctionArms and GunBroker.com) but they are a bit more complicated to deal with, I think, and if I were to want to sell somthing of that caliber, I'd check them out first. The Gun Runner auctions get good exposure.
Selling it will require that you obtain a copy of the buyer's FFL license or Collector's license and there are rules for shipping a rifle that you'll have to follow. If you need directions about that, post here again and we'll walk you through it. Hopefully your buyer will be up on all the paperwork necessary. This is a firearm and there are strict laws about how one ships and to whom it is shipped.
That's pretty much it in a nutshell.
The Johnson is one of the "holy grail" of collector rifles to us and even finding one to buy is near impossible. So, when they do come up, they get a lot of attention.
Finally don't do ONE THING to that rifle. Don't polish it or clean it. Don't scrub the bore, don't turn a screw. Don't even cock the charging handle. Let it stay just as you have it. Nothing you do to it will help the value and could easily harm it. Knowing collectors look beyond all of that stuff.
You're a lucky guy and I'm sure a college student like yourself could use a nice piece of change in your bank account. So, putting it up for auction to the world will both help a collector fill a big void in their collection and put money in your bank. And, probably most importantly, the rifle will be preserved for the future.
Good luck!
Rome