The water parameters for Betta are not specified because they can tolerate a very wide range. Hence people keeping them in jars with nothing but dechlorinated tap water.
However if you are looking for ideal parameters, I would keep them in soft water.
That is
pH: 6.5 - 6.8
Temp: 25degC or 78degF.
KH: 3-5
GH: 6-8
TDS: doesn't really matter. These guys can live in crap water (not that you should aim for crap water). But I would aim for under 350 like for all my other softwater fish.
Most important thing is to give them a tank that is well cycled and has low flow. They do not prefer a strong filter and flow. Some floating plants will also help them build bubble nests if you have females (note the plural. 1 female is not enough and will get harassed). If you are putting the betta in a 15L, then I would avoid females in the same tank until you see him building a nest.
Apart from that, they are considered very easy fish to keep.