There are two types of front on (one on one tackles), drive & block.
THE DRIVING TACKLE
when approaching attacker keep your body square, keep your eyes on target area (mid section). Dont bend to tackle to early, you need to get in close to attacker and dip late,when bending legs to tackle keep head up looking forward, if tackling with right shoulder your right foot should be in between attackers feet or as close as possible, your left foot forward at the right side of the attacker, your right shoulder driven into mid section, your right arm wrapped around the back of the attacker, your left hand behind attackers right leg just above knee.(your head should still be looking up and pressed against the right side of attacker hip area.)Lift attackers right leg with your left arm and drive forward by straightening your legs, you should land on top of the attacker.
Opposite positioning of feet, head, arms for left shoulder tackling.
THE BLOCK
Approach attacker the same way as the driving tackle.
Hit the same target area, with head and feet in the same position. The arms should be wrapped tightly around both of the attackers legs.
(keep head locked in tight to attacker hip area, chin up)
If tackling with right shoulder use attackers momentum and fall backwards and twist to your right (whilst still staying locked in tight and arms wrapped tight around defenders legs) so you end up on top of attacker and your head does not end up between the attacker and the ground.
The last couple of steps (for both styles) when approaching the attacker are the most important, when coming into the attacker shorten your stride, stay square, come in on a slight angle if possible and dip late.
Practise these techniques by using a friend or team mate, first by moving into the attacker from only two steps away walking and alternating shoulders, you can then increase the distance and pace as you become more confident. The next step is to get the attacker to start trying to evade you when you have mastered the technique.
There is also a smother tackle but I'll leave it for another time, hope this helps.