There's much about these fish on the forum. But in brief: these are quite hardy animals but prone to being territorial and possibly snappy, so are best kept alone or with their own kind. The exact salinity doesn't matter, but aim for at least SG 1.005 for adults (about 9 grammes marine salt mix per litre). Keep the temperature around 25 C. Omnivorous; wild fish eat plants, algae, snails, crustaceans, insects, and (reputedly) the fins of other fish. In captivity they'll eat most anything meaty, as well as tinned peas and often algae wafers.
The fins should heal readily in brackish water, but it'd be a good idea to use something like Maracyn or eSHa 2000 to prevent finrot setting in. Melafix is often suggested but frankly I know a lot of very experienced fishkeepers who consider the stuff worthless, so I wouldn't recommend it.
If you have a freshwater tank already, your prime problem will be cycling a new tank for this puffer. Set the new tank up as a freshwater system, and use some media (up to 50%) from the other tank to immediately mature the new filter. GSPs are fine in freshwater for months, so there's no problems setting the tank up thus and then slowly raising the salinity. Begin by doing 25% water changes each week adding SG 1.002-1.003 water, and then after 2-3 months raise the salinity in the incoming buckets of water to SG 1.004-1.005. This approach won't shock the filter bacteria.
Cheers, Neale
PS. There are multiple species sold as Green Spotted Puffers, but the Latin names used for them are not reliable, so it's best to work in generalities as far as care and maximum size go. Do see here:
http
/homepage.mac.com/nmonks/Projects/FAQ/3e.html
