Welcome to the forum! I always reccomend grey tree frogs to new frog owners for three reasons.
1) They tolerate a wide range of temperatures and humidity levels.
2) They almost always eat well, whereas others frogs are easy to go off feeding when they feel stressed.
3) I am biased. I own 5 (soon to be 6) grey tree frogs, and I absolutely love them!
Here are my wet skinned compadres:
^ From left to right: Bud, Rocket, Kermit, Weiser, Frogger
No matter what species you choose as your first, you will surely love the hobby and find everything you need here on this forum.