Question

What in a cell tells it what to do? :)

Answer 1

The nucleus which contains all of the cell's information does.

Answer 2

When talking about cells it can get complicated so if you think about the cells like a team, more specifically a baseball team. The coach would be the nucleus, it trains the cells/players. It provides plays/or ways to defeat viruses. The players would be the white blood cells, the one's fighting the enemy virus/team. 

Your answer would be: nucleus... (the coach)

Hope this helps, I like to create scenes too help me better understand things.