both net speed and direction will change or only the direction will change -- depends on the way force acts.
If the force acts at one instant perpendicular to the velocity of the particle, then the direction of the particle changes. The speed also changes due to the impulse (force * time duration it acts). The resultant velocity will be more in magnitude (vector sum of the two perpendicular components).
If the force acts continuously at right angles to the velocity of the particle, then the particle goes around in a circle. This is like the planetary motion, Earth moving around the Sun. This is called uniform circular motion.
Since the change in velocity is always perpendicular to itself, it contributes to change in the direction only. The force supplies the centripetal acceleration. The speed and kinetic energy of the particle remain constant.