I have no idea what app I purchased first, but if I had to guess, I'd say it was either Tapatalk or a weather-related app....I think...
If you open iTunes and go to your account page (click on the button with your Apple ID on it and go to Account) you'll see there's a Purchase History area...click See All and you'll be able to then jump to the earliest entries.
For me, it was Multi-Photo, back in 2009: Multi-Photo solves an iPhone problem | TUAW - The Unofficial Apple Weblog.