If the sun had a gravitational pull, how come the sun doesn't suck in our planet?

The linear or tangential speed of the movement of the Earth, in its orbit, prevents that the Earth from being absorbed by the gravitational pull of the Sun. Actually the Earth is falling toward the Sun continuously, but the trend to continue in a rectilinear motion does not allow its fall towards the Sun.

The first law of Newton (inertia law) proposes that a body is kept in rectilinear motion, with constant velocity, when there are no countering forces acting against this. If the Sun did not exist, the Earth would have a rectilinear motion through space. The Earth is continuously forced to change the direction of its movement (into an orbit) due to the attraction of the Sun.

So the tendency of the Earth to maintain a rectilinear movement, in a tangential direction to the orbit, is what prevents its fall towards the Sun.