I've hired many, and recently even have had to interview a few.
I cannot emphasize strongly enough the need for a spec to go by.
Now since you are not a programmer, you'll NOT want to tell them much about *how* you want something done. It is both fair and prudent to demand a certain language so you can be certain to be able to purchase help later. PHP is an excellent choice. If a database is required, use MySQL.
But since you cannot define much more than that, you'll want a gorgeously clear storyboard of what you want coded. With as much detail as you can muster - even copy and pasting from example sites to make your point clear. Make the pages of your storyboard look just *exactly* the way you want your program written - at least as much as you can. If you cannot even do that, then you'll want to take this in three steps.
First: Find anyone you can trust to work with on an essential outline of what your app should do. This person should be strong enough to help you with what can be done, what can't be done and what shouldn't be done. Decide if you want it to run on Windows, or Linux, or the Mac, or a web application.
Second: purchase someone to create your storyboard. This should be a working relationship until you have exactly what you want your program to do. "Walk" the program ie., imagine pressing each button... do you have a page in your story board to show you what happens when that button is pressed? You should be able to "see" the entire program running in your head and storyboard before you have anything written.
Third: Get AT LEAST 3 quotes for it - I'd personally not go with less than 5. You may post some highlights here if you want help. The quotes should be human readable and describe things in such a way that you feel comfortable enough to gamble your hard earned cash on someone you have no experience of, building something that you don't understand, via an agreement for a sum that you have no idea if it is actually worth it.
Good luck man.
Then again, there's "Teach yourself PHP in 24 hours" ...