I have a few questions about DC motors, would anyone be able to answer them?

how does current flowing through the coils generates a magnetic field

How does Switching the current change direction of the field

as the coil rotates the commutator changes the direction of the current

 

The questions are a little vague, DC P.M. or wound field motor? The current in the armature produces a magnetic field from the iron core it is wound on.
The field in the armature reacts with the field whether the field is P.M. or wound, the strength of the field and applied voltage decides the rpm of the motor.
The motor also generates a BEMF which opposes the applied voltage and will increase to a level which will almost equal the applied voltage, this will decide the resultant rpm level.
For more insight study Flemings right and left hand rule.
Max.

 

let's assume a DC permanent magnet motor.

Your first two questions can be answered with Ampere;s Law: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/amplaw.html

Without a commutator, the shaft would get "stuck", It effectively says move here and when your here, move to the next position, so the motor rotates.