The US Patent and Trademark Office (
http://www.uspto.gov/) is pretty easy to use. You should check it out before doing anything else.
You can find registered patent attorneys through the website too. There's a guide to let you know the steps you should take (
http://www.uspto.gov/web/patents/howtopat.htm). You can also do a patent search to see if the item you want has ever been patented.
If nothing else, write your idea down in a notebook. When you're done writing, go back and number the pages 1 of X, 2 of X, etc. and on the last page mark an giant "X" to fill the white space. Go to a notary, sign and date it and get it notarized. You don't have to mail it to yourself.
I know a patent attorney (I do work for him once in a while) and I know he's talked to a few people who haven't had good experiences with the "invention kit" people.