Japanese is not as clear-cut as the previous answer suggests. Although "ai" is the most general term meaning "love", it depends very much on the context etc., which term you would use. The meaning of "ai" can range anywhere from "charity" to "passion", For example, "aichaku" means "love" in the sense of "attachment", "affection", whereas "ren'ai" means "love" in the sense of "tender passion" or "amour if you want.
Thus, you might want to specify your question.
To tell someone "I love you", however, you would say "aishite iru" or even "aishiteru". Another possible expression is "suki" (I like you) or "daisuki" (I like you a lot)