Here are the laws of the game: 

Rugby Primer is a good outline of rugby laws, skills, strategy, and fitness

The Advanced NZRU Primer for the advanced player and club administrators.

The Laws of the tackle Part 1  Part 2  Part 3 is a great series for the tackled player on how to navigate the tackle and post-tackle and for the support player(s) on how to approach the breakdown.

Footwork into contact is a clip that illustrates what the tackled player should be striving for when contact is inevitable.

Competing at the breakdown is a good illustration of the need for support in the tackle, as well as quick attack of the ball on defense.