告白(こくはく/kokuhaku), literally means “confession”, and it is done when a man or a woman declares their love to another, and hopes to begin dating that person. The most basic way of confessing this is to say:

好きです。付き合ってください。 すきです。つきあってください。 Sukidesu.Tsukiattekudasai.

This translates to “I love you, can we start seeing each other? The 付き合う(つきあう/tsukiau) part means “dating”, “seeing each other”, or “having a relationship” in English. This is a very common phrase used for this kind of confession and you may have heard it, or a phrase similar to it, once or twice in Japanese movies or anime. If accepted, it marks the beginning of a “serious” boyfriend/girlfriend relationship. Like real grown-up stuff.