How do you take your opponent's weapon in combat? I couldn't find rules about it in the books. There is a chapter about "knocking a weapon away", but this isn't what I want. I want to grab the weapon and take it from my oppenent's hand. What rules do you use/suggest for this kind of situation?
This can only be done with an unarmed attack (which may mean an off hand if you're doing this while otherwise armed), by grabbing the weapon directly and attempting to wrench it away from them. The procedure is short and detailed on B370 under "Grabbing": a called shot that, if successful, is followed by a ST contest. The defender defends normally, for which the procedures on B400–1 for defending against a disarm are a useful reference.
Being unarmed combat against an armed foe, it's obviously rather dangerous and you should either take appropriate precautions (either during combat, or when creating the character), or only attempt this in a Cinematic game.