With respect to children learning an instrument, the inherent desire to want to play MUST come from and be instigated by the child. Do NOT force or push a children to learn an instrument, if the desire is not there.

If you are starting out as a kid, make sure the teacher is well schooled in music theory, and can communicate theory concepts in a way a child can understand it. Learn theory.

If you plan to make a career in music learn theory. It is invaluable especially for improvisation and opening up new concepts and ideas, as well as for doing arrangements for not only your instrument but other instruments.

Learn to become proficient in as many different styles of playing as possible. You'll increase your ability to find gainful employment as a musician.

Seek out objective opinions on your abilities from those you respect, not those who are inclined to patronize you (family, friends, etc.) Be realistic about your abilities, and attempt to improve upon them all the time. If you have been playing for a few years and are not improving at what you're doing, find another outlet and resign yourself to the fact that you just don't have it - unless it gives YOU pleasure to keep on doing what you're doing.

If you are blessed with God-given talent, learn to be tolerant and patient of those with lesser abilities than yourself and a develop a sense of humility . (Arrogance is a major turnoff, no matter how much you might excel at playing.)

Further, if you do plan on music for a career, (and no one can dissuade you), you have to want it more than anything else in the whole world, and be able to achieve a sense of self satifaction just from playing/performing.

Unless you are one of .2% who luck out with a record/distribution/concert deal, the music business will not give you back a tenth of what you give to it...so it's very important to "want it". Learn to love Kraft dinner and peanut butter sandwiches, and the simpler things in life.