Bettas will often eat crushed up flake food, you can get betta pellets if your lfs sells them too. Bloodworm is usually accepted, but don;t feed more than 2-4 worms, and not everyday as that can make the fish bloated.
A 5 gallon ( about 25-30 litres ) tank is a safe minimum size tank for a betta. If you are new to fish then getting a smaller tank is not only really bad for the fish, it's also a LOT more work for you to keep the water in good stable condition, , which will not be helpful to a newbie. The easier the tank is to keep the more successful you will be at it and your fish will live a lot longer.
So, a cheap 5 gallon tank ( try getting one second hand, it doesn't have to be new, and will cost you a lot less ) ,
Betta DO require a filter like all other fish , and a good sort of filter to have for a betta is a sponge filter. You will need an air pump to run the filter, but both are pretty cheap to buy and you can get both in most fish stores and also on ebay.
Bettas also need a heater. you should set it to 27 degrees C, which is pretty much in the middle of the reccomended temperature range for bettas.
Lastly, bettas prefer a LOT of plants ( as in 10-15 plants or more depending on the size of your tank, not just one or two plants ) . If you don't have time or paitience for live plants, then get lots of SILK ones ( not plastic, they can cut the long fins ) You can buy packs of them on ebay, and the more you have the better for the fish. They like to have lots of plants to hide in and rest on which is what the wild bettas do.