1. 5 American gallons, which is 20 litres. I usually keep mine in 25 litre tanks, which seems to be quite a
common size.
2. It does need to be cycled and heated. Bettas like it warm, so no less than 25
oC. They can be kept in unfiltered tanks, but having a filter makes it much easier.
3. If the tank is filtered, 25 to 30% once a week, though by the time I've cleaned the whole tank, I find half the water has gone.
If the tank is not filtered, 50% every day and 100% once a week. This is why I said that filtered tanks are easier.
4. They need specific betta pellets as their staple diet - Hikari is the make easiest to find. Live food, (or frozen live food) once or twice a week - I've found my bettas have all liked bloodworm and brine shrimp. If you have just the one fish, frozen live food is easier as you'll waste a lot buying bags of actually alive, live food. With the frozen cubes, you just cut off as much as the betta can eat and defrost it.
Be careful about overfeeding a betta. They will beg for food whenever they see you. Don't give in. They are very prone to bloat if overfed and will stuff themselves to the point of bursting if they have too much food. Brine shrimp does help clear them out, as does a cooked, deshelled, chopped up pea.
Fasting a betta for one day a week is also good for them. Fish do not eat every day in the wild.
5. Some of the bettas in shops are in very poor condition. If they are in small cups or tiny little tanks, look for a betta that is alert. If you hold your finger up to him and move it round, he should follow it. His pectoral fins (the ones by the gills) should be moving all the time.
Check his fins for ragged edges. Avoid fish that are covered in white spots, have a sunken or swollen abdomen, or have fins that are obviously disintegrating. Don't buy a fish with white/cloudy eyes. And don't buy a fish that just lies there on the bottom of the cup.
If the betta is in a tank with other fish, check all the above but also make sure he has not been attacked by the other fish as damage can be hard to heal. If he is cowering in a corner, he could just be trying to keep out of the way of the other fish, or he could be sick.
6. Decor in a betta's tank needs to be smooth or those long fins can get torn. The simplest way is to take an old pair of tights with you and run them over the potential purchase - if they snag, it's too rough. Though your user name sounds like you are male, so perhaps take a female companion to do this
Batteas do like large leaved plants to rest on just under the surface. These can be real or silk, but not plastic as they tend to have sharp edges. A cave of some sort - coconut shells are good for this. You can buy them from a lot of fish shops and some have java fern attached to them.
Bettas do tend to get bored if their tank is too bare - lots of plants (real/silk) and things to swim through are good decor.
As for what to put on the bottom - if you want real plants, I'm not much good with them. others will be able to advise on what to use. If you don't want real plants, or just something like java fern which grows tied onto decor, then gravel, sand, whatever takes your fancy on the botom