Question

Why were the colonists against the British when they were charging them taxes? (For a class "argument" and I'm against this idea)

Answer 1

The Colonists were not represented in the British Legislature. They had no say in how taxes were levied against them. 

Answer 2

The colonists were against the British because the British was putting these crazy laws on them [Quartering act, Tea Tax etc. ] and would not give them land that they helped fight for in the French and Indian war. Basically the colonists felt they were being treated unfairly in this deal with the British