Oh wow....okay, we're going to want to start from the very beginning here.
First of all....kudos to coming to the forum and starting on doing some research! Your new little guy will really thank you for it in the long run. Those horrible vases with plants are basically torture chambers, set up for nothing more then a horrible short life and a slow death by starvation and poor water quality. They really should be banned....its extremely sad that they seem to have recently made a come back again. 8/
I'm sorry you ended up with a betta like that, but at least you took the initiative and want to make his life better, which is fantastic. :)
Now, this is all going to be a little over-whelming at first.....
So, first of all...like the others mentioned, bettas do not eat plant matter and can't really digest it well anyway. They are carnivorous and need a diet high in protein with meaty foods. You are going to need to actually feed your fish.
Here is a thread about choosing commercial foods and feeding and such: http://www.bettafish.com/showthread....99#post1263699
The example of the better food used is Omega One, but due to a lot of controversy over the brand recently I would recommend New Life Spectrum brand instead. Its a higher quality anyway. But the thread explains the basics.
Since we're starting from the very very beginning here....well, there is a LOT of info. I could probably spend all night telling you everything I know that could possibly be helpful, and even then I'm sure plenty of members could come in with a bunch more information. Please take a look at the stickies in the Betta Care section. There is some great basic info there, and some of the questions asked in those threads might be questions you have/will have after reading so take a gander at those too.
Here is the basic guide to pretty much most of the basic info on the forum:
Also, here is a thread that several members who ended up reading through it informed me of how helpful it was for a beginner.....so it might be a bit of a read, it is several pages, but a lot of beginner keeper questions you may have might be in here. Just if you want to give it a glance.
Best of luck! And of course if you have any more questions, always feel free to ask. :)