In Japanese, confessing your love is called “告白 kokuhaku”. Confessing your love is never easy, even so if you’re going to say it in a different language. However, he/she may be touched by your effort to say your love in his/her language. The most basic way of confessing is to say
好(す)きです。付(つ)き合(あ)ってください。Suki desu. Tsukiattekudasai.
I like you. Can we start seeing each other?
付(つ)き合(あ)う tsukiau means “dating”, “seeing each other” or “having a relationship”. It is a common phrase used for confession. You may have heard of it in Japanese movies or anime. 好(す)き suki can mean either “like” or “love”. For confession, using suki will suffice, instead of 愛(あい)してる aishiteru (I love you) because you do not know whether the feeling is reciprocal and also, you do not want to creep her out right from the start.
Good luck with your confession!