How do you say I hate studying in Japanese?

If you want to but nice and say "I dislike studying Japanese" it is 日本語の勉強が好きじゃにです。　'nihongo no benkyo ga suki janai desu.'

But if you want to say hate (which is rarely used and is like proclaiming your deep loathing of something) you can say 日本語の勉強が大嫌いです。 'nihongo no benkyo ga daikirai desu.'