Teenager looking to get into gardening!

Hi! I've been wanting to get into gardening as a hobby and was wondering if anyone could help me figure out where to start? We moved into our house a few years ago and in a corner of the backyard is a greenhouse/shed type thing left behind by the previous owners who I assume were pretty big on gardening. I want to use that but I'm unsure what I can plant in there? Its not very big, but has two shelf type areas that can hit plenty of pots and such on them and a trough-like planter in it. Here are a few pictures!

The Greenhouse/shed - It needs cleaned out and the door fixed :P Imgur

Shelf 1 - Held made of cinderblocks and a wooden board. Would this be stabe? Or should I try to fix it up a bit? Imgur

Shelf 2 - Not sure what its held up by... Imgur

The trough Imgur

Pots on Shelf 1 Imgur

Big Orange Pot on Shelf 2 Imgur

Spare Cinderblock - Maybe I could plant an herb or something in it? Imgur

All of this stuff was left behind by the previous owners. Any advise on where to start? The only thing I'm really set on growing is grape tomatoes, but I'm not sure if I can do that in here? I don't want to really go outside of the greenhouse as we have a very crazy puppy who digs like crazy. If I can do grape tomatoes in the greenhouse/shed, what varity is good for a beginner? Also, wheres a good place to start researching gardening as a whole? I want to learn as much as I can before starting! I am also thinking about growing flowers as well. Will that be okay in the greenhouse/shed? I'm in Northwestern Montana, USA by the way (4a on the zone map?)