Answer:
Well first of all, find a Stage Coach or singing teacher that you can see often (once a week is best). It doesn't matter if it is a group or private, if they do show music or musical theatre, this is a good start. Then, try out a few musical theatre exams, and enter a festival under the show songs category and competitions. These will build you confidence, and give you scope to what other people think of you performing. Audition for shows (AUDITION AUDITION AUDITION, MORE AUDITIONS!) and try out anything, even pantomime! DO what even you can to gain a portfolio and expierence.
After a while, if you think you are good enough and you love it more than anything, get an agent, and they will do what they can to get you roles in musical theatre.