First, how gross is the couch in question? Has it been sitting outside, getting rained on and moldy? Have the filling/cushions rotted out or disintegrated? If we're talking about a couch that it is gross because it is moldy and rotten, or because Heaven forbid it has bedbugs, I'm going to say throw it out. Sorry. Some things are not salvageable. Remember the episode of Portlandia when Daniel and Meg go dumpster diving? I'm all for upcycling, but there's a limit, and that limit is any food that is unsanitary and couches that are literally rotten. K?
Now, if your couch is merely gross because the upsholstery fabric is ugly or dirty, then by all means take a stab at reupholstering it. First of all, it could be worth it to take your couch to a professional upholsterer. If that's out of your price range, then follow these tips, many of which I got from Amy of While Wearing Heels and All Things Thrifty.
1. Start by taking photos of the couch with its gross upholstery on it. You'll need them later. Take note of all of the details of the couch. Which parts of the upholstery are sewn? Which parts are stapled? Document the way the the couch is constructed well so that you can recreate it. The upholstery on the cushions will most probably be sewn. Do you have a sewing machine? Do you know how to make patterns? The most challenging piece will be recreating the cushion covers, as it will require sewing.