This is not to be considered as legal advice. In the US there is first to invent. So any ideas you have, you need to write down, sign, date and have witnessed by someone who can not be considered an inventor. This will establish your date of invention. If you have publicly used, sold, or offered for sale, your invention this establishes day 1 of a one year time frame within which you need to file a patent, provisional or otherwise. The consequences are that you can not patent this particular invention.
A provisional patent application cost $110 to file, for a small entity. This establishes a filing date. You have 1 year to convert to a non-provisional application. The problem arises in that everything that is written in the claims of the non-provisional must be supported by the specification of the provisional if you want (need) the file date of the provisional. If the claims are not supported, and it can be established that you used in public, sold or offered for sale more that 1 year prior to the file date of the non-provisional, you can not get a patent, or if you do get a patent, a potential copier can show that your patent is invalid, and you've spent all this money for nothing.
So it is somewhat tricky in that a patent attorney (or patent agent, less money) will know these things and be able to protect your idea. You can write the provisional yourself but you are running the risk that the later patent is invalid. If you are serious about this, I assume you have a business plan including methods of getting funded, which needs to include IP prtection. As Henry alluded to, you need to run a prior art search to ensure your idea has not been patented. Just because you can't buy something doesn't mean it is not patented. Your other, major concern, is that software patents are coming under a lot of scrutiny in view of a case called in re Bilski, which is a case that will determine what and how software can be patented, relating to 35 USC 101, is it patentable? This is where an experiences agent/attorney comes in handy and earns is money. I hope I didn't dissuade you, but these are some things you need to consider.