STEP ONE is to go room by room, drawer by drawer, piece by piece and decide which things are deserving of a place in your life. Everything you own takes precious time and adds clutter to your life and space. If you use something, then it is worth the time to keep it up. If you love something it is also worth the time to clean around. Only you can decide whether something makes the cut or not.
If you don't use something or love it, then get rid of it. If it is trash, then trash it. If it can be used by someone else, then you have another decision to make. You can either try to sell it or give it away. If you have the time and inclination - then go ahead and try to make a little dough. If you don't, though, don't feel guilty and store things 'in case' you have time to sell it in the future. Pack it up and donate it to someone who needs it. I fall into the second category. I used to feel guilty that I was giving away something that maybe I could sell . . . but I then realized that, for me, it wasn't worth the time and energy. So, I let go of the stuff AND the guilt. Again, that's a decision you have to make for yourself.
Once you make the initial purge, your job isn't over! When you live in a small space you must continually weed things out. I've heard some people get rid of something for every thing they bring into their house. I just try to go through things several times a year and ask myself, 'is this still useful to me? Do I still love this?'. If the answer is no, then out it goes!
So, take your time. . . and start purging. Try to just work for 15-20 minutes a day. You'll be surprised at all the STUFF you purge from your home and your life.