If you want to say you're for something, you use за. If you're against something you use против. On to examples!
за - for
против - against
быть за — to be in favour
быть против — to be against
Ты за? - Are you in favour [of it]?
Кто за? - Those in favour?
Все против. - All against.
Ты не против? - You're not against it? Do you mind?
Ты не против, если я возьму это? - Do you mind if I take this?
Я за мир во всём мире. - I'm for world peace.
Я против войны. - I'm against war.