This is pretty much a summary of everything I know about bettas, I hope it helps. The good news is that, despite the length of this post, a betta set up can be pretty simple. I'm just trying to answer your questions as completely as I can.
No, it's not true that you cannot keep more than 1 per tank HOWEVER, BEGINNERS SHOULD KEEP ONE PER TANK. Leave it for the more experienced to try more than one per tank - it can work, but is a bit sketchy. Both male and female bettas are generally aggressive to other bettas, so it's much safer to just keep one betta per tank.
You should use a heater if the spot you put the tank in varies in temperature more than a few degrees, or is stable but too cold. For example, I have a spot near my computer that stays warm and stable, but anywhere else and the tanks definitely require a heater. Even for the tank in the single warm stable spot, I still use a heater, just to be sure. There are heaters to fit every size tanks, even 2.5 gallons. For small tanks, heaters are not terribly expensive, so I think it's worth it. You'll have to look up (or someone else will tell you) what the best temperature is, because I don't remember.
No blue stuff is needed. What is needed is regular water changes with a good water conditioner. Make sure you use something that neutralizes chlorine and chloramine, and follow the instructions. When changing water, make sure you clean the gook out from the bottom of the tank (fish poo and left-over food accumulates in the gravel). Use water that is close to the temperature of the water the betta is in. You can buy siphons ("gravel vacs"), which pull out water and gook (if used correctly), but they're kind of big an awkward for small tanks (2.5 or 5 gallon tanks) so a 10gallon might be a better choice. Do NOT just "top off" evaporated water.
Basic betta set up:
Filtered, heated water, with plants. If you're not comfortable with caring for live plants (they certainly add a lot of complexity to an otherwise simple setup) , opt for fake - but make sure to buy silk and not plastic ones. The plastic plants can rip a betta's fins.
You can find a heater and filter to fit pretty much any size tank.
A betta set up can get much more complicated than this, but doesn't need to be. Since this will be your first betta, I would recommend keeping just one betta and no other fish. There are a number of small fish that can be kept with bettas, but trying to find out which and deciding what to get and such just complicates things.
Bettas breed by building bubble nests. You can buy a male betta almost anywhere (and females are much more available than they used to be). If you buy a male betta and he starts leaving large piles of bubbles in the corner of the tank or against a floating plant - it's perfectly normal and many consider it a sign of a happy betta. He will do this regardless of the fact that there will be no females (or other fish at all).
A healthy betta will flare at other bettas. In most stores, the bettas are stacked in cups next to each other. If it looks listless and is not interested in the other bettas around it, it's probably not terribly healthy. Google "flaring betta" if you don't know what this looks like. If it's building a bubble nest in it's tiny little cup, it's probably good.
