Drastic change before school?

Okay, so im becoming a freshman in highschool and I really want to get a hair cut or hair dye, I want to dye it darker. But I have Puerto Rican hair thats very thick and I straighten a lot. Ive never dyed my hair before except for highlights(thin ones) which I did once. Im afraid if I dyed it all over that it would become very brittle and harder to manage. Would it do that too if I got thick highlights? help please.