It is difficult to apply Mendalian genetics to bettas since many traits are on the same chromosome (which is something Mendal did not know when he made his thesis). However in general:
VT: Dominant (D)
Round Tail: D
Crowntail: CoDominant/Incomplete dominance: in other words, if you cross a CT with a VT/HM/SD you will get a VT/HM/SD with (not so nice) spikes
Single Tail: Semi-Dominant to Double Tail: when crossing a DT and ST you will get both tail types.
Double Tail: Recessive (R) you should not cross two DTs, only cross DT x ST
you will get some DT and some ST fry
HM and SD are basically recessive to VT and RT... HM and SD have more of a gradual inheritance pattern. If you cross SD x SD you will get a whole bunch of SDs. If you cross a SD and a HM you will get mostly SD and some HM. HM xHM will get you some HM and some SD...
Colors are MUCH more complicated. Here's a journal I wrote on it awhile back, it isn't perfect http://news.deviantart.com/article/139935/