Rythym is just the chords to a beat with a particular strum pattern.
What style of music is it? HMetal, rock, country?
What's the beat? 2/4, 4/4,3/4?
What's the tempo? 120, 80?
How do you feel about the girl?
What's her first name?
Have you dated long or is this to get her attention?
All these need to be considered when writing good lyrics.
I usually start with the lyrics and try to sing the words. That's when the melody comes to me. Then you work on the beat and tempo. Different strum patterns determine the style.
I mean anyone can write, "I love ya baby, I need you baby, I want you baby"
but that's not a good song.