A good begginer bike for a 14 year old?

A bike like the ttr250 would be a good starting point for a beginner or look at the crf230. Certainly do not get a 2 stroke for a first bike especially if you only intend to use it for trail riding. They are dificult to learn on and are crappy for trail riding for a beginner as they need to be ridden in the power or they foul plugs and over time develop other problems. It is better to start off with something usable and then move on once you are comfortable and have gained some basic bike skills