I don't use kits anymore. I did for many years, but now I just buy the ingredients ala carte. Stick with kits at the beginning, and as you gain experience, you'll gain an understanding of what the various ingredients add to a brew, and you can then venture out into making your own recipes. That's my opinion anyway.
When I *did* use kits, I used the "Brewer's Best" brand (available from many brew shops, online and in-stores), but I know a lot of guys use kits from a lot of places. Austin Homebrew has some kits that people are very pleased with, so they would be good ones to try (if you order enough, you'll get free shipping, too.)
Irish Moss
What it is: dried seaweed
What it does: you add it with 15 to 20 minutes left in the boil. It rehydrates and helps proteins in the beer clump up together and fall to the bottom of the kettle. It helps make the finished beer more clear (homebrew tends to be a little cloudy.)
Where you get it: any homebrew shop or online retailer will carry this.