Bring up the subject to your parents. Test the waters. Ask them for it.
You have not for you ask not. If they flip out on you, then you have more convincing to do.
Reason rather than whine. Parents want to reason with you. They want to hear from you a legitimate reason for them to buy you an expensive instrument. Unless you get a really cheap guitar, guitars are really expensive. It's a big purchase.
Tell them that you're going to play it, practice on it and that it will be worth the money that will be paid.
If you really want the guitar, you have to express extreme interest in it. Download articles, music sheets, tablature and even paperwork on the guitar that you want.
Offer to help finance it (if possible.)
Once again, if you're serious, don't let one 'no' let you down. Maybe your parents are testing your persistence. They'll be the ones buying the instrument so act like you won't take "no" for an answer. Keep asking. Not bugging, asking. Keep reasoning with them and sending the signal that you're not joking.

Warnings

If you are successful in getting the guitar, do not not play with it for a month and then let it sit in your closet for the rest of your life. That will prove to your parents that you're really not serious about your big wants and it will make them seriously reconsider the next request.
So don't ask for the guitar unless you plan on actually using it.

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