So you are experiencing engineering - you have to design the whole system to select individual components . . . and each individual component affects the whole system. Its very overwhelming at first but gets easier as you learn.
I find the easiest way to build a system is to divide it up in to smaller isolated subsystems. For example: power regulation circuit, amplifier circuit, microcontroller circuit, motor driver circuit, etc. Then connect them all together.
So what do you want this transistor to do? (you forgot the specs!)
and engineering was never a game of luck.
there was once my circuit went up in smoke . . . and then when I turned it back on it mysteriously still worked and I couldnt find anything wrong . . . some luck involved
and an exception to the magic smoke rule . . .