How Can I Get A Career In Musical Theatre?

Like a career in any area of the performing arts, musical theatre is hard to get into. It is a competitive industry with lots of talented and hard working people. Your reputation will always precede you.

The most important thing is – Training! You need to have;
a strong, healthy and well maintained voice,
a healthy, flexible and well maintained body
a high level of dance training
a high level of acting training
stamina
perseverance
confidence and enthusiasm

I cannot stress how important it is to look after your voice and body. You are your own instrument, your own tool and the thing you are selling! If you don't look after yourself you will have a very short career!!
You should get appropriate training at college, uni or preferably a drama school. Take outside singing and dance classes. Take any opportunities that come your way.

If college/uni is a while off then you can still take private classes and join an amateur group for experience.