You have a green spotted puffer. Read about them here: http://www.aquariumadvice.com/forums...le-174144.html
Your puffer is swimming up and down the glass since there seems to be nothing in the tank. If you fill it up with decor, then your little fellow will stop glass surfing. Stress may also make him glass surf.
Your tank will eventually have to be converted to a saltwater tank for the duration of your GSP's life. Ok to leave him in freshwater as a youngster for a few months.
If you just leave the snail in the tank, then your puff should be smart enough to figure out that its food.
Here are the things you need to do:
1. Make sure your tank is cycled and have enough beneficial bacteria.
2. Examine your puffer for any external abnormalities for a few weeks. Treat for bacterial or fungal infections if needed.
3. Treat for internal parasites by using Prazipro or Jungle Meds. Puffers are very prone to internal parasites.
4. After your puffer is healthy for a month, then you can start planning to turn your tank into a saltwater system. Post more when you get to this stage.
Also post if you have other questions.