So you bought an entry level instrument for your beginner and they've been taking lessons for awhile, they've learned how to take care of it, they've gotten used to the dimensions of the instrument and stopped bumping it on everything around them. You want to upgrade their sound without spending a fortune and buying another all-out guitar.
First, the best way to improve your sound is to practice. Much of a player's sound is in their fingers and how they use them. Eric Clapton will sound like Eric whether he's plugged into a $100 Squier or his one-off ~custom~ custom shop Strat (yes, I said it twice). There are countless stories of stars who reached the dressing room, found a cheap piece of junk to warm up on, and made it sound like a million bucks.
Did you practice?
The second best way to upgrade your sound without spending a fortune is to upgrade your pickups. Pickups are the electronic, magnet-containing gizmos under your strings that pick up the vibrations and send it through your controls to the amplifier. There are thousands of pickups out there and they all can sound slightly different than each other, even the same model by the same company.
The way to approach a pickup decision is to reverse engineer it and do your homework-- find a sound you like, experiment with the products they use, and find your own sound. Be careful though-- every pickup company wants you to look at their endorsers and choose your pickups from there. That's a good start but not the best way. Those pros have thousands of dollars and hours invested, and hired more pros just for their sound. So take that information with a grain of salt, and then go to youtube and find regular, run of the mill average Joes that have made videos of them using the product. Their sounds are going to be closest to what you can get, so make your decision from there.
When you find the stuff you like, have a pro install them-- it's well worth the money to have it done right. It might take some experimentation to find the combination you like, but the YouTube approach will get you in the ballpark. With the prices for mid-level instruments these days, it's a valuable alternative.
Email us if you're in the Minneapolis & St. Paul area and you're looking for a reputable repair man, we know the best. firstname.lastname@example.org