A good guideline is: Spend a little more than you can afford.

In The Schoole of Musicke (published in England in 1603), Thomas Robinson wrote:
Student: "Why, mee thinks it were no great matter what instruments a beginner hath, considering that for the most part, young beginners soone make old instruments, as young scholars soone make old bookes."
Teacher: "Indeede to have a good instrument lieth as much in the habilitie of the parents or friends, as also in their good wils to have it so; but why I think it good to have (if it were possible) even the verie best instrument for a learner at the first, is this: a good instrument will please a learner every way, for it delighteth them to looke and beholde it now & then, likewise they love easie and smooth instruments, and although they can do but little, yet it will be found well, and so incourage them to learne with delight, whereas contrariwise, a bad or dull instrument will quell their spirits quite, so that in a long time, or never, will they profit in their forced labours."
Student: "... againe it is an old and true saying, that one good thing is ever worth ten bad, also there is small losse in a good thing, it ever yeldeth monie with profit ..."

If you choose a used instrument, set aside some money to have it inspected and adjusted. Properly adjusted, quality instruments don't just sound better, they feel right and are easier to play.

Once you have an instrument, learn to care for it properly.

Much of the advice for choosing an instrument also applies to choosing a teacher (but hopefully your teacher won't require factory servicing). Teaching and playing are separate skills and not everyone does both well. The best performers sometimes make poor teachers because they learned too easily. They never had to think about how they play or why they do it that way. The best teachers will at least play decently, and usually play very well. Bad teachers often pass their bad habits and poor technique to their students, either with wrong advice, or because they don't take steps to prevent it. The real test of a teacher comes when the student seems to hit a brick wall and just stops advancing. A good teacher can help you rethink the problem and explore alternate solutions, ultimately helping you over or around the wall. Bad teachers often create problems that must eventually be unlearned before progress can resume. No one teacher knows everything, but if you have more questions than your teacher has answers, it may be time to seek a new teacher, especially if your progress is slowing.

Work with your teacher to set realistic goals. Do you just want to learn a few simple songs or reach your maximum potential? You're probably capable of much more than you realize, but will you put in the necessary time and effort to play your very best? Music should be fun, but fun and accomplishment can go together. How do you feel when you finally succeed at a difficult task? How do you feel while working toward a goal? Do you perceive each small step along the way as a success in itself? Do you enjoy the journey or just keep asking, "are we there yet?" Learn to understand the many small accomplishments that make the journey as enjoyable as getting "there", a place that often moves away as you approach.

For some, just learning a few simple songs can be reward enough, but there is also a danger in taking too many shortcuts. If you accept whatever happens as "close enough", you risk picking up bad habits that won't interfere much at first, but can become formidable obstacles to further advancement. Unlearning bad habits and relearning everything can be even more frustrating than taking a little extra time at the beginning.

Teachers work for you, but you must be willing to trust and accept their advice. Help your teacher by discussing the hard parts. Ask specific questions and listen carefully to the answers. Tell your teacher where it hurts. Tension or pain may be a sign that you're doing something wrong. Playing well feels easy and natural. Playing badly is often a struggle because muscles get tense and different muscles may actually work against each other. The hard part is learning to play well. Practice slowly and carefully. Relax. Isolate the hard parts and work on them separately. Perfect practice makes perfect. Sloppy practice makes sloppy. If you've worked diligently but still can't do it, it's probably time for another lesson. Find out why you're having problems and if you should do something differently. Don't use slow progress as an excuse to skip a lesson. A lesson is not a performance and the teacher isn't a paid audience (although teachers usually appreciate hearing the results of your hard work). The teacher's primary job is to help when it's less than perfect and to provide some motivation and encouragement as you work toward perfection.

Don't worry about "talent" - most people create their own. Patience, persistence and attention to detail usually take you much farther than someone else's idea of "talent" or "ability".

People tend to hear what they want to hear, or what they think they should hear. When singing or playing an instrument such as violin, some of the sound vibrations actually get into your inner ear through bone and connective tissue as well as from your eardrums which changes the sound. If you have access to a quality recorder, use it. The first reaction is often, "Something is wrong with the tape recorder!" Learn to hear yourself as others hear you.

Non-sectarian meditation promotes physical and mental relaxation and improves your ability to focus. Many communities offer classes, or informal sessions in a private home. Ask around, and learn several techniques.

There are many good books available. These are some I've read that contain information applicable to any instrument. Some require reading between the lines and a little effort or imagination to apply the principles to a specific instrument. If you can't find another book that's exactly what you want, one of these may be close enough.

Betty Edwards, Drawing on the Right Side of the Brain , J. P. Tarcher Inc., 1979
From the jacket: "This book can teach you to draw even though you feel you have little talent and doubt you could ever learn."
My comments: This book presents some valuable fundamental concepts, and is well worth the effort and imagination required to apply those concepts to playing an instrument. Recommended.

Barry Green with W. Timothy Gallwey, The Inner Game of Music , Doubleday, 1986
From the jacket: "It is specifically designed to help every musician overcome obstacles, improve concentration, and reduce nervousness, thus paving the way for heightened performance."
My comments: Written by a fine double bass player, this book presents some valuable concepts, but I generally prefer Sports Psyching (see below). The principles in this book were derived from W. Timothy Gallwey's book The Inner Game of Tennis . Both books are worth reading.

Kato Havas, Stage Fright - its Causes and Cures With Special Reference to Violin Playing , Bosworth & Co. Ltd., 1973
My comments: As this book was written by a fine violinist with a very interesting technique and teaching style, the exercises are for violin (or similar instruments), but the general principles also apply to other instruments. Worth reading.

Yehudi Menuhin, Violin - Six Lessons with Yehudi Menuhin , Viking Press, 1971
From the Acknowledgments: "... to the man whom I sometimes call 'my best violin teacher', Mr. B. K. S. Iyengar, my yoga guru ."
My comments: This book was written for fiddlers by a highly-respected violinist, but the general principles also apply to other instruments. Worth reading.

Thomas Tutko and Umberto Tosi, Sports Psyching - Playing Your Best Game All of the Time , J. P. Tarcher, Inc., 1976
From the jacket: "How to overcome the tensions, fears, and frustrations that undermine your game and keep you from winning."
My comments: Change "game" to "recital" and "playing field" to "stage", and you've got some valuable tips for musicians from a psychologist (and very entertaining speaker) who has worked with many professional and college athletes. It doesn't take much effort to apply the principles in this book to music. Highly recommended.