I've probably done 10-20 projects with zippers and I would say that for each one that turns out great, one turns out so poorly that I throw it away. I buy zippers in bulk now.
If you are trying to make zippered pouches, buy the pattern from Oh Fransson:
http://www.etsy.com/shop.php?user_id=5192619 Seriously it's worth the $10. I wasted more than $10 worth of zippers before I found this pattern.
The EASIEST way to have a quick success is to not center the zipper. It's much easier to make the front zippered pouch without doing a separate lining, but it won't look as polished since the edges are raw. If this is just for you, and you don't care, just zig zag the edges and be done.
Like in this tutorial:
http://boltneighborhood.typepad.com/bolt-neighborhood/2009/07/zippered-coin-purse-tutorial-from-sally.html Or this one:
http://javajem.blogspot.com/2008/02/wristlet-tutorial.html(There are like 10 of these tutorials so if you don't understand, google "front zip pouch tutorial" and you'll get a bunch.)
-- I wouldn't just leave the interfacing on the inside though - you can do this with 2 layers of fabric and it looks nicer. For the most part the edges get tucked in and you don't see them, especially if you use a layer of batting, so just zig zag and you'll be fine. I have an "overlock" stitch on my machine that I use because I don't have a serger (well I do, but it's a hand me down and I don't know how to use it!) and it works fine. It looks like 2 straight stitches and one zig zag. As someone else said about a longer zipper - this method also works best with a longer zipper! Honestly, I've done it both ways and if it takes me 2 hours to make the Oh Fransson pouch it takes me 30 minutes to make the front zip pouch. I'm also working on a way to make it without having the raw edges, so if I figure that out I will share!
I've never tried to do clothing or pillows with zippers...
You could also do a zippered pocket:
http://u-handbag.typepad.com/uhandblog/2007/02/zippered_inner_.htmlI also use the wonder under stuff. I actually found that 1/4" strips of heat & bond lite work GREAT for keeping the zipper where I want it. I've done a few of these so that I can zip my wallet up in my purse but my purse doesn't need to zip. Much easier

It's OK. I was where you are a few months ago.

Erin