I guess you could try heating the tank with a 25w but it'd be a lot better if you could get a bigger tank. You'll need to keep a very close eye on the temp since it can fluctuate easily in that small of a tank, which is stressful to the fish. I highly recommend getting at least a 5gal if possible...the betta will be a lot happier and it is easier to maintain a larger tank.
You should definately get test kits and a thermometer. The temp should stay at 80. Here's a link explaining the nitrogen cycle
http://www.fishlore.com/NitrogenCycle.htm You'll need test kits to test your ammonia, nitrite and nitrate. The best kits are the
API liquid tests-get either the Master test kit or individual kits for the three (the other tests that come with the master kit are nice to have but you probably won't need them). Since you just got the betta the tank probably isn't cycled, and you may see high levels of ammonia and nitrite. It's important to test these since they can kill fish. It may be necessary to do water changes more often till the tank's done cycling.
As for food Top Fin isn't the greatest brand as far as nutrition goes...the best ones are Omega One and Hikari. The best commonly available betta pellets are Hikari Betta Bio Gold pellets. It's also nice to give them more variety than just pellets-a lot of bettas like bloodworms (hikari brand is best). The amount you are feeding is good.
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