Mailing something to yourself doesn't really hold up in court. If you want to verify that you had an idea on a certain date, write it down, sign and date it, and then get someone else to sign as a witness. This is how the classic laboratory notebooks are done (many of which are electronic now, but that's a different topic). If you want to be absolutely sure, get a notary to stamp it.
But what does this get you? Just proof that you made the invention before someone else. It prevents anyone from taking your idea and filing a patent on it. It does not give you any rights to the invention. An easier way to do this is to publicize your idea.
As Jason pointed out, any patent filed after March 2013 will be under a first to file system. Therefore if someone can prove that they developed the same invention independently of you, and you hadn't made your idea public, they can still get the patent and stop you (and everyone else) from using it.
If you do find a pro bono patent attorney to help you write the patent, they can help you with the search as well.
What you can do now is to file a provisional patent now for $125. A provisional patent has many many less formalities involved (such as formal drawings), and you can do it yourself (after a little education). This gives you a year in which to find someone to file formally for you (which costs only $380 but there may and probably will be other fees involved). You will also get the benefit of the first-to-invent provision and an earlier filing date.
The US Patent Office has a lot of help on this - the link is below. Just rememer, getting the patent is the easy part (but beware of scams). Making money from it is harder. You need a business plan. But you can do it!
http://www.uspto.gov/inventors/independent/index.jsp