Leonardo
He asks you out by bringing you flowers and out right asking you.
Donatello
He asks you out by leaving a note in your locker.
Michelangelo
He asks you out when you're hanging out together.
Raphael
He asks you out after you stumble upon his brothers teasing him and him blurting out for them to knock it off about his crush on you.
April
She asks after your first day of school with the brothers.