Grumpy_Mike, what do you mean by " the -ve terminals should be connected together " , it will be like above picture ?
Yes.
How much voltage and amper Arduino and motor shield solo itself handle ?
No idea, have you a web site link it will say there.
Also the data sheet for the shield should answer the other question.
what kind of dc motor I should use with how much rpm I need
Not sure what you are asking. Recommending any one motor is a bit useless unless it is accessible for you. The RPM is not a parameter that affects the torque so I don't know what that has to do with it.