Handball is a sport where a certain amount of contact is permitted.
To tackle properly, you contact the opponent's shooting arm at the upper arm or the shoulder to take away the opponent's shooting power.
The other hand is placed on the opponent's body at hip height in order to control the opponent's movement.
Both arms must be bent and you must be at a standstill by contact.

After you contact your opponent it is not permitted to push the opponent away.

It is important that you are in total balance when contact is made, as the opponent will try to get past you and you must follow the opponent all times and maintain your position between the opponent and the goal with very fast and short sideway movements of the feet.

The difference between tackling and blocking is that by a tackle you attempt to prevent the opponent from shooting.
When blocking, the opponent is able to shoot and you try to place one or both hands in front of the ball.