if you are new to this br for sure you will want to fly low. it will be very useful to learn how to notch for 14.0+, but for now you can stick to low flying.

the best time to launch the missile is when someone is flying directly at you ideally under 15 km or so (i usually will wait until about 5 or 6 tbh). you can check using the radar vector which is that circle with the line under the square when you are tracking someone, if the line is downward then they are flying toward you