which type of transistors are preferred in this situation, BJT or
MOSFETs are usually preferred because they are driven by voltage and have low on-resistance. Just make sure the FET can be fully turned on by the voltage from the Arduino. Choose 'logic level' FETs, or those which are specified for a Gate drive of 4.5V or less.
Bipolar transistors are current amplifiers. To stay 'saturated' (fully turned on) you need to feed ~1/20th of the Collector current into the Base. The Ardiuno's PWM output can only deliver 20mA reliably, so a typical small power transistor may only be able to switch ~400mA (even if rated for 2A or higher).
I can easily set the voltage and current to the values that I need
Transistor voltage and current ratings are the maximum that the device can withstand under controlled conditions (no overshoot, large heatsink with good cooling etc.). For reliable operation you should increase the voltage rating by at least 50%.
For the current rating you should calculate the power dissipation and temperature rise from current draw and on-resistance, or just select RDSON using the rule-of-thumb that it should drop less than 0.1V at the desired current (in your case 0.1V / 2A = 0.05Ω).
You will also need a 'flyback' diode across the actuator, to recirculate current caused by the inductive kick that occurs each time it is switched off. Schottky diodes are usually used because they switch fast and have low voltage drop. The diode should be rated for at least half the peak load current. Without this diode the actuator will produce very high voltage in an attempt to keep the current going, which will quickly destroy the FET.
how can I set the value of switching frequency of the transistor.
Your PWM frequency is quite low, so just about any transistor that meets the other requirements should work.