Magnus is a great ganking and teamfight hero that has a vacuum and a stun built into his ultimate! He also has a large variety of aoe magic nukes that allow him to turn the tides of teamfights very easily.
You should go mid lane, pick up a bottle, and farm the lane with nukes and eventually purchase an arcane boots(my favored boots) and blink dagger and then transition into a lot of ganking.
I sometimes gank early if I have the right runes as well.
Late game pick up Refresher, BKB, Force staff, and things like that!