+1 to majesticstorm, really hit it on the nose :)
You can try looking for a small filter as well to help you out with keeping the aquarium clean. Tetra makes a great little filter called Whisper In-Tank Filter 3i. It's for up to 3 gallon aquariums and isn't windy and it's in take will not hurt your Bettas fins. You can find this filter at WalMart, PetSmart, PetCo and i'm sure your local fish store will also have it.
What type of water conditioner did you buy? Some are much better than others and always always check the expiration date on them, I've heard horror stories from people who have bought expired conditioners and accidentally burned their fish. If you want a really good conditioner try looking for API Stress Coat+, it's amazing stuff. It's a water conditioner as well as a stress reliever, it also helps with your fish's slime coat.
Flakes are 'okay' some fish prefer it to pellets and some prefer pellets to flakes. It's really easy to over feed your Betta with flakes and they also tend to cloud the water. Omega One has pretty good pellets but my Bettas don't seem to care too much for them, NLS is great and is made from higher quality food. I mostly use Aqueon Betta Pellets for my guys, it's never failed me before and my guys love it. It's fairly cheep and is pretty good for them too, not super high end but it's still great.
Yes, silk plants they are a must have for Bettas. Now, if you're getting a female or a Plakat you can get away with having a few plastic plants so long as they aren't rough, however silk plants are still way better. Also, if your getting decorations make sure they don't have metal or they will rust and cause a whole ton of other issues. All plants and decor should be for actual aquarium use, if you get something that is painted and not meant for an aquarium the pant can come off and yeah, it's not good.
Before adding anything to an aquarium, including gravel, it's smart to rinse it all off under a running tap this will get rid of any debris that they might have picked up while sitting on a store shelf.