Baby steps. That's what you need to remember. Start out by getting yourself 3 big bins (or boxes, whatever) and label them KEEP, DONATE, TRASH. Now, grab ONE of your many boxes in the study and open it. Go through each piece and decide RIGHT THEN if it's a keeper, a donater or a trasher. If it's something you use often, obviously it's a keeper. If it's something you haven't used in over 6 months (aside from off season clothing) then donate it. If it is broken, doesn't work, etc, trash it.
Paperwork...you need to keep your important papers for 2 years. After that, shred them and recycle the paper. Keep the ones you do need in a nice file cabinet or file box in the garage or somewhere out of the way.
As far as knick-nacks and gifts from relatives. I don't keep things JUST because they were a gift (except for certain items that have SPECIAL meaning to me like antique quilts my grandmother and great aunts have made, sentimental items from my kids's baby days, etc). If it doesn't have SPECIAL meaning to you, toss it in the donate box.
Now, once you've gone through all these things and there are still several things you aren't sure about, put them in a box and date the box 2 or 3 months ahead of the current date. So, like if it were today, date the box October 31, 2008 or November 31, 2008. Put this box in the garage or somewhere out of the way. If you realize you need something out of the box before that date, go and get it and store it where it needs to be. If you haven't used things in that box by that date, donate the entire box. You obviously don't need those things b/c you haven't used them in 3 months!
Hope the ideas help, and if you need more ideas on organizing specific things, let me know...I'd be glad to help!