It depends on the relationship because in many situations, we use だいすきです or すきです more than あいしています. I would say: わたしもあなたがすきです。(Watashi mo anata ga suki desu.) However, generally it is used between close people, after someone tells you "I love you", isn't it? So we say: わたしも！If I wouldn't omitting nothing, it is: わたしも(name)がだいすき！(Watashi mo (name) ga daisuki. *We generally use the name instead of saying あなた. *A lot of women use あなた when they call husband. I hope this was helpful.

"boku mo kimi o aishitemasu." ぼく　も　きみ　を　あいしてます You could drop 'boku mo' since it is already implied that you are the speaker.