I like u = watasi wa anata ga suki desu. I hate u = watasi wa anata ga kirai desu.

Ryo's answer is good. Another way of saying it: Sukidesu = I like you Kiraidesu = I hate you. Please be careful, if you say Sukidesu, it can also means I love you. If you want to say I like you, not I love you, then the best way to say it will be "Ki ni irimashita".

watashi wa anata honto ni ga suki = i realy realy like you anata wa ichiban no kirai desu yo = you are the most i hate

i m not sure about the form but this is how we can say i like u = anata wa suki desu i hate u = anata wa kiray desu