there is a lot of ways to say it. Usually situational. But the simplest is "Mahal na mahal ko siya" meaning I love him/her very much". Which is usually enough to depict that you love the guy with all your heart. Saying the whole sentence like "Minamahal ko siya ng buong puso ko" meaning I love him/her with all my heart is usually not being said not unless it is in a poem, a court injunction, a song or some formal applications. Tagalog is usually situational. A different phrase will fit a different situation. And please note that Pilipino has some differences from Tagalog. Pilipino is the national language and Tagalog is a bernacular or dialect. They are not entirely the same.