Hi i want to make very simple gear shift mechanism in creo how can i do this???

Mechanism constraints.

That is the simple answer and the starting point.

I will assume you already have all your parts defined.

With assembly constraints, you can provide various limits rather than hard constraints.

In your case, you will have a Pin constraint, for instance, for the shafts.

Pin constraints require an axis, a position, and optionally, an angle.

You also need a slider constraint.  Know that you can add constraints.  Meaning, you can constrain the Pin to provide rotational motion where you leave open the "translation" direction of the Pin constraint undefined.  You then add a Slider constraint, and you can put limits on the linear motion of the shaft.

Additional mechanism constraints can be added with gear connections.  This is how your motion will be synched up between the shafts.