first up: what is the enclosure that you bought/its dimensions. This will severely alter the type of reptile you can keep.
Next is; consider what it will need as a grown adult. A bearded dragon will need a vivarium that is a minimum 48"x18"x18" - I recommend wooden as you also want to retain heat and a glass fish tank will leak it faster than Julian Assange.
Throughout its entire life, a bearded dragon will also need a source of UVB (replace every six months, £15+ depending on what type of bulb you choose) & a basking spot, meaning hot lamps so many "starter" enclosures such as a faunarium are useless. If its an exo terra glass cube, it is also pretty much useless for snakes & dragons.
childrens pythons are not something I would recommend for a begginner - they can be very very fiesty and viv defensive which can put you off handling them very quickly, which will make the issue all the worse.
Green tree frogs (and any other frog) are pretty much "essential" handling only, but they require something tall as well as wide/deep. A 12x12x12 exo terra is a good starter, but you will need to increase to something more like a 24x12x18 as it/they grow. they can also escape the tiniest of gaps, dont consider these for anything less than an exo terra style glass viv.
Depending on your enclosure, other alternatives you may want to investigate:
Lizards:
Leopard Gecko
Crested Gecko (these guys are just amazing)
Snakes:
Royal pythons - these can be picky eaters and for a beginner you may worry about them not eating. stories of them refusing feeds for 12 months + abound BUT thats not to say you can just relax when they refuse feeds. when stricking, these make the most seasoned feeder jump sometimes: they strike faster that your eyeball can update your brain with a new image. handling these buys, they like to just curl up around your arm and sit there.
Kingsnakes - florida/cali -- these guys can be full of character, but respond well to perisistant handling & like to move around when being handled, Going to other types of kings can be hit or miss on character and attitude.
Cornsnakes: any: very popular amongst people starting in the hobby who then wish they got a royal python! they move around like the kings when young, but as they get older become much more docile although nothing like a royal.
If you can post details on the enclosure, happy to give more info