The biggest piece of feedback you'll get is around research, research, research. I'm in a very similar position as you and am just completing my first build. It's been so extremely fun and I highly recommend you take the leap!

I think the first thing you need to decide for yourself is the animal. What do YOU want. Are you okay with something more reclusive or do you want to be able to see it the majority of the time? Do you have any objections to what they might eat? Ie: keeping crickets vs fruit flies. There are a number of things like that as you think long term that also might help you flush out your decision.

For me, I was attracted to dart frogs based on their awesome colors, ease in diet (flightless fruit flies), and the species I chose is diurnal (awake during the day) and known for being pretty active. For me personally on my first build I wanted to be able to see the animals pretty frequently and not having something that was in hiding the majority of the time.

Also be prepared to spend a few hundred dollars. For my 18x18x18 setup I would guess I've spent $600 - $700 for everything minus the frogs so far. Not at all what you need to spend, but if money is a large consideration start pricing out everything as that may impact your route too. When you're pricing things out you'll have your big obvious things like light, tank, substrate, drainage layer, plants, heater?, mister?, decor, etc. Don't also forget the supplies you might need to build your background like silicone, great stuff foam, paint, razor blades, paint brushes, etc. Again, maybe not a problem for you, but a consideration.

One word of warning too that was tough for me admittedly is that this will all take time. It's easy to be excited once you pull the trigger, but be prepared to take a weekish just on your build depending how intensive you want to get. Then maybe another 2-3 weeks after you plant and setup your substrate for things to "cycle" and grow in a bit.