It depends on your Vds/Vgs/Vgd/Vbs/Vbd voltage.
The process has maximum allowable limit. For example, if your max voltage difference is 10V, then you need 10V process
what do u mean by "how to choose"?
Do u mean there are lots of transistors type you can choose?
In my experience, there are high VT transistor, medium VT transistor, low VT transistor and even zero VT transistor