Don't know which transistor I should use
OP schematic shows only the basic H-bridge. If logic gates, or a micro-controller is to switch the H-bridge to one state or the other, extra driver transistors are required. Designing for the motor's 6V/1.5A stall is a robust way to start, but keep in mind that motors are not usually meant to remain in a stalled state for long. 

For the four big power transistors (two PNP, two NPN), choose at least a three-amp transistor...perhaps complementary pairs like MJE170, MJE180. Since these will be working hard, their saturation voltage will mean that the motor will not see the full supply voltage. The two NPN driver transistors (Q3, Q4) needn't be so big - a plastic generic 2N2222 is sufficient - something in the 0.5A class.

 The 22 ohm resistors should be at least 0.5W: they'll get warm. Don't forget the large electrolytic capacitor with a short path from PNP emitter down to the big NPN emitter.
My generic LTSpice only had some Rohm transistors in the 3A class, so that's what were simulated. From the 6V supply, the motor sees 5.47V, the rest appears across Q2/Q5.
It is assumed that a 5V microcontroller or HCMOS logic signals will drive the 2700 ohm resistors...don't allow them both logic"1" simultaneously, else smoke. This is about the simplest bipolar H-bridge possible.