A Front Kick (also called a Front Snap Kick, Flash Kick, or 앞 차기 ap chagi) is performed by first raising the knee of the kicking leg so that the knee is aimed at the target. The ankle is held straight in-line with the rest of the leg. The leg is then straightended to kick. After striking the target, those same actions are performed in reverse: the knee remains pointing at the target while the foot lowers, then finally the knee is brought downward as well.
In Kukkiwon/WTF-style poomsae and in ITF-style taekwondo, the toes are bent upward during the kick, the idea being that you strike with the bottom of the balls of your feet (somewhat like a Push Kick). For other applications (such as WTF-style sparring), the toes are pointed straight in line with the rest of the foot, the idea being that you strike with the topside of the foot to extend the reach of your leg.
The Non-Kicking Leg:
For all taekwondo kicks, what you do with the off-leg is as important as what you do with the kicking leg. For the Front Kick, the off-leg should be bent slightly at the knee during the kick. Your body's weight should be on the ball of the foot rather than the flat of the foot; the heel may even be lifted slightly from the ground (but not too much). The off-leg's foot should rotate during the kick so that at the moment of impact the off-leg's foot is turned partially away from the target. (It shouldn't be pointed entirely away from the target however. That much rotation is reserved for kicks such as the Side Kick.)
Torso and Arms:
The torso is normally tilted back somewhat during the kick, with the fist brought to the chest as if blocking. Often to provide a "reaction" force to the kick, the arm on the kicking side will be brought downward and back as the kicking leg is brought forward.