A "What's up?" text from someone you're interested in is a low-effort opener that puts the conversational weight entirely on you - how you respond determines whether the exchange dies or develops into something real.
Turning the talking stage into a first date means reading conversational momentum accurately and making a specific, confident ask at the right moment - not waiting for a perfect opening that may never come.
What to text after she agrees to a date is about maintaining the right level of presence - enough to stay real, not so much that you become predictable before you've even met.