तु (tu) is only used for a very close person. तुम (tum) is probably the most widely used form of you in Hindi. तुम can be used for everybody unless you want to be polite, which you would then use aap (आप).

Isn't tu more for close friends or someone you want to insult? I learned that you have to use tum for younger people or people you are good friends and aap to older people and strangers...or am I mistaken?

you are learned all forms for grammatical purpose here. however the practical use of tu, tum, aap depends the hierarchical position of the person to you and how polite you want to address to that person.