The first thing to do is to find an instrument you enjoy playing. To keep on buying new instruments until you find the one you like can be very expensive. So I suggest joining a school band and speaking to the school music teacher to see if you can get extra time using some of the schools instruments until you find the one you like.
Once you have found the instrument you enjoy playing the most you need to practice on it as much as possible.
Affording some musical instruments can be very expensive so to afford your own can take you some time of saving so you may need to look for a part time job to bring in extra money.
If you can’t afford to buy an instrument yourself using your local library and school music teacher research and see if there are any grants locally or nationally that will help you buy an instrument.
Again if you have the money search out local professional music teachers who will give you weekend or evening tuition.
Once you feel confident you can start taking musical grading examinations which give you a nationally recognised grade and tests your competence at your musical instrument.
Try your hand at as many styles and genres of music as possible to survive as a professional musician you need to be versatile.
Playing at family events like a family gathering at Christmas can be a great way to practise performing.
Join as many bands as possible and offer to play in as many assemblies and school productions as possible.
If there is an area local to you where it is legal to busk this can be a great free learning experience of performing in front of a live audience.
Find as many places and events to perform at as possible from local church fetes to hotel lounges all will gain you great experience and great exposure.
Research the best colleges and universities in the country to study music at and apply.
When you feel ready start recording your work and start sending your demo to prospective employers like record companies or local venues.
When you have bookings coming in or being asked to play on various projects start your own website which advertises what you are doing and that gives the chance for people to sample your music.
Keep an eye on music publications, local and national press for news on auditions for groups or adverts for musicians needed.