Oh, wow.. where to start? You should have done the research before getting the fish, but we'll clear this up.
You need to read this.
Read ALL OF IT. This will give you everything you need to know about your new betta.
But let's cover the basics.
1st, 1.5 gallons isn't really big enough, but it will do for now. If you can get something 2.5 gallons or larger, do so. Even one of those little critter keepers at the pet shop and Walmart will do.
2nd, Betta are tropical fish. Just giving him warm water won't do it. It has to STAY warm. Around 78-82 degrees warm. You want to get a heater, or keep him in the warmest room of your house.
3rd, your conditioner should have directions on the back. It should say how many table/teaspoons per gallon, or have a number of drops. Just go by that. If you don't understand it, I suggest asking someone in your house for help, or, if you have to, type out the instructions here and we can read them and help you.
4th, The water he came home in should be conditioned. You'll have to condition the water you are setting up for him. You'll want that water to set. The bottle of conditioner should say for how long it needs to set, but if it doesn't, I would wait at least 3 hours before you put the fish in. You'll want to "float" the fish before putting him in the new tank water. Keep container you got him, keep him in the container with the water he came in, and let it set in the water for a while so he get's use to the water temp. If you put him in right away, he might go into shock. You want to put a little bit of the new water from his tank into the cup every 15-20 mins so he can get use to the new PH. When the cup is overflowing with new water, he's ready to be released into the new tank.
5th, what kind of plant do you have? And you might want to make sure the coral doesn't have any jaggad or sharp ends on it. A betta's tail is very delicate; Plastic plants, sharp edges on decorations and sharp rocks can cause rips. Keep your fish safe and he'll be a lot happier.
6th, FOOD! You need to feed him 1 pellet at a time. Watch him eat them, and clean out anything he doesn't eat. EVERY SINGLE TIME. A betta's stomach is about the size of his eyeball. Most people feed their betta 3 or so pellets a day. I feed mine 2 in the morning and 2 in the evening. But they're like dogs, they don't know when to stop eating, so watch him, and make sure he doesn't over eat.
7th, You will HAVE to clean his tank often because it is so small. change 100% of his water twice a week, and between those, change 50% of his water. That should keep him very clean and happy. If you don't, things like Ammonia, nitrites and nitrates can build up and make your fish sick! Never use cleaners to clean his tank. Hot water should do.
8th and last, if your fish is new, he might hide a lot and refuse to eat. He needs to get use to his surroundings. He'll be fine for a week or two if he's still scared, but if he's not eating after that, you may want to improve his habitat. And hiding is normal, once he gets use to you, he should come out more.
Now, how long have you had your betta? And are you caring for him alone, or are your parents helping?