Good to see someone saving themselves so much trouble! Jumping into fishkeeping without researching is nothing but bad news, wasted money & dead fish.
I will warn you beforehand never to take the advice of LFS (local fish store) employees. They rarely know what they are talking about and I have seen a lot of well-meaning people come to this board having wasted lots of money on merchandise that is unsuitable for bettas or useless.
You may have heard that letting water 'age' eliminates chlorine. Until recently fishkeepers were not aware that aging does nothing to eliminate chloramines, heavy metals and other compounds that are a part of the growing list of crap cities are adding to their water. So even with aging your water you would still have to add water conditioner. Since water conditioner eleminates everything, you might as well do away with aging and just use conditioned water from the tap. I recommend Seachem Prime. Prime will remove any ammonia in your tap water as well, and if you have a big ammonia spike you can add 5x the recommended dose to get things under control until you figure the problem out. Since 1 millilitre treats 10g you should only need a very small bottle.
Live plants... as a beginner you need to look for 2 qualities in your plants:
1) They are actually aquatic. Some pet stores sell terrestrial plants as aquatic plants. They can survive submerged for a few months before they die and rot in the tank. Huge ripoff.
2) They only require LOW light. If you buy a high-light loving plant it will pretty much wilt unless you have a special, high-wattage lighting fixture.
Some really great plants to start off with are java fern and any anubias, cryptocoryne or moss species. They are all quite low-maintenance and thrive in beginner tanks.
As far as diet goes I recommend you base the diet around 2 or three quality pellets such as Hikari Betta Bio-Gold, HBH Betta Bites or Atisons Betta Pro, then supplement with frozen or live foods such as bloodworms, brine shrimp, krill or tubifex (I only buy Hikari Bio-Pure Tubifex; tubifex worms are notorious for carrying internal parasites but Hikari sterilises their well).
I also strongly suggest you complete a fishless cycle before you put your betta in. This will colonise bacteria that eliminate the need for 100% water changes. Here is the jist of cycling:
Rotting food and fish waste produces a toxin called ammonia. This is usually what kills fish when the tank isn't cleaned often enough. Given enough ammonia, bacteria called Nitrosomonas start living off it and starting a colony in the tank. They live on all the surfaces, primarily the filter because it contains the most surface area. They eat the ammonia and produce a less-harmful waste product called nitrIte.
When nitrIte builds up, it attracts another kind of bug called Nitrobacter bacteria. They do all the same stuff as Nitrosomonas, only with the nitrIte instead of ammonia. What comes out the other end is a chemical called nitrAte. It's only harmless in big amounts. You then vacuum the nitrAte out of the tank when you do the water change. It doesn't take alot of vacuuming to remove.
In an uncycled tank, you skip the bacteria by sucking all the ammonia up yourself. Since you are cleaning out all the ammonia before the bacteria get a chance to dig in, they never really colonise the tank. So you take care of all the ammonia yourself, which requires bigger water changes to eliminate.
AMMONIA >nitrosomonas bacteria> NITRITE >nitrobacter bacteria> NITRATE => Water Change
I hope that helped and welcome to the forum!