During foreplay, when highly excited, how do you know when girl is ready ?

You could ask her. Being considerate is a great thing, and this way you are getting her verbal consent. You can make it sound sexy if you want. Yes, most people do missionary - even after the first time.

Go slow. Make sure you use lubrication. Make sure she is 100% ready and highly aroused. If she is lubricated, aroused, and relaxed, there should be no pain or bleeding - even her first time.

If she's going "oh, God, yes, take me" that's probably a pretty good hint she's ready. If she's texting or yawning, probably not.

I'm not aware of any statistics on what position most people do their first time. Choose something that works for you both. It doesn't matter.

you will know, and i didn't do missionary first time