How to know the right amp to power a 12V actuator?
Without details like what actuator you bought or a datasheet it is a little difficult to give you exact information and guarantee its correctness.  However we can do a bit of educated guessing.  

If there are only 2 wires going to the actuator, note not 2 control wires but 2 wires total then we have to assume thats for power and ground.  Power them one direction and the actuator moves power them another and it moves the opposite direction.  We can determine the maximum current the actuator can consume by measuring the resistance between these 2 leads.  12volts/resisitance(in ohms) will give you the maximum current draw.  So if you measure 1 ohm then the current draw is 1 amp for 12 volts.  

This is way to much for the arduino to handle.   If I were setting this circuit up I would use whats called an H bridge.  However using relays might be a bit easier as a beginner.  You will need 4 relays that way you can turn either wire positive or negative.  Depending on the relay you probably will not be able to trigger it with the arduino directly... Hook one side of the coil to your 12 volt power supply and wire a logic  level n channel mosfet up to the the other. Put a resistor between the gate of the mosfet and your microcontroller pin.  That will allow you to turn the relay on and off with your arduino