I wouldnt worry to much about selecting a trade. They will probably ask you during your training anyway, however if not then you can ask then. Just worry about getting fit and staying healthy. The rest will fall into place.

Hello,
Just to say I joined at 21 (many years ago!!), I would say that the perfect age.
Make sure your very fit when joining, it makes it easier through basic training, most of the punishments will be excercise, it really helps!! Have a good all round strength, speed, endurance fitness. Be ready ...