Hi Nataseya, welcome to the forum.
There is a section where you can find quite some picture's for the enclosure:
section
You can see lot's of picture's of how others decorated their cages. As for the size: Garters are active snakes that use the size that you give them, so I'd always go for a quite nice cage. My cages are 100x50x50 cm, but if I had the room I would prefer 120x60x60 cm. This is certainly big enough for a trio.
For the food, I'll direct you to the new wiki

:
If you're wondering about the food, you've probably never heard of thiaminase, so this is quite interesting to know:
Thiaminase
This is a global thread about food on the wiki:
general caresheet
I can certainly advice you to use the search function for words like red earthworm, fish, steak, pinkies, ...
I hope this gives you an idea, feel free to ask some other questions.