First You need to decide how big/small You arm is going to be.
Start sketching base of the robot in solidworks with dimensions in mind.
Design each part of the arm from base going up, leaving some placeholder space for motors.
Think about material Your arm will be made of. Assign that material to relevant parts of the drawing.
Now, You should get fairly good estimate of how much each part will weigh.
Use parts' weights and expected maximum load to calculate minimum torque required, double it.
Find motors with desired specs. In motors' datasheets You will find drawings of them. Add these drawings as separate parts to Your SolidWorks model, and there You have it.