You need to know the stall current of your motor before choosing a transistor.
Rather than go through that effort (which generally requires study of electronic engineering), why not just build a circuit that has well chosen parts (Q3 and Q4) and is known to work, like the following?
For NPN and PNP, just about any bipolar transistor will work, but 2N3904 and 2N3906 are old favorites, respectively.