Choose the transistor based on the stall current of the motor. The stall current can be several times the running current. The stall current should be listed in the motor data sheet. If you have no data sheet it is possible to estimate the stall current, measure the motor winding resistance. Take several measurements rotating the motor a bit between readings. Use the lowest reading in the calculation. The estimated stall current is the motor supply voltage divided by the measured resistance.
You need a logic level MOSFET. That is a MOSFET that can be fully turned on by a logic level signal (5V for a 5V Arduino, 3.3V for a 3.3V Arduino). You can tell a logic level MOSFET by the Vgs specification in the Rds(on) specification. The Vgs should be logic level signal voltage or lower.
Do you want surface mount or through hole package?