When embarking on any big project, the first thing to do is commit to the hilt and tell everyone you know. Tell your friend, the one with the pit-bull tendencies that will question and question you about it until you finally say, “Okay! Fine! I’ll do it today!” Tell people on Fluther that every time they see you they have to ask, “So, how’s that book going?”
Make sure you have tons of people around you to remind you of your goal when you feel lazy. I’m incredibly bad about dreaming up ideas and lacking the follow-through, but if I tell other people of my intentions, I feel an obligation to get it done. If I don’t, I live with the shame of lying or misrepresenting myself to my friends.
And since your collection is scattered everywhere, the first thing to do would be to go through it all. Block out several hours (or multiple several-hour sessions) to get all your stuff together and begin reading through it. You don’t necessarily need to type it all up into one folder right now.
Find the things you’d like to include, or the things you feel you could polish and incorporate into your collection. Spend a few days looking those over, editing them, and so on. A good way to stay motivated at this point is to ask a friend to bug you every three days or something if you haven’t emailed a new piece of writing to them. This is, again, shifting the accountability from yourself to your friends.
When you’ve edited the pieces you like (or culled the ones that you’ve realized are unsuitable) start looking at them and figuring out how to organize your stuff in a book. You could divide it into two sections: poetry and prose. You could divide prose even further if you wrote some memoirs, some short stories, and miscellaneous flash fiction. You could organize chronologically and see how your viewpoint and writing has evolved. There are quite a few options.
The last part (putting it all together) requires the least thinking but the most busywork. I think the easiest way would be to format it in Microsoft Word, unless you have experience with a page layout program (InDesign). There are also quite a few companies that help you publish nice bound books. I’ve heard good things about Blurb, although it seems to be primarily for photos. If you’ve ever done NaNoWriMo you might have a coupon for Createspace, another POD company.
(I write too much.)