Part of being a guitarist is the proper care and setup of your instrument. Guitar students that have 30 minute lessons weekly and practice for 1-2 hours a week should change their strings at least every semester (August and January). The older your strings get, the more grimy and difficult your guitar will be to play.
A basic guitar setup includes setting the action, adjusting the intonation, and cleaning. Lost? It's not as bad as it sounds.
Setting the Action
The "action" refers to how close the strings are to the fretboard. The farther the strings are from the fretboard the harder you'll have to push down to get a good sound. Lowering the action helps guitarists to achieve a better sound with less effort.
Adjusting the Intonation
This step involves getting the guitar in tune with itself throughout the length of the string. While I hope my students are tuning every time that they play (right guys?), setting up the intonation is a little different. Once the guitar is tuned, screws on the saddle (on an electric guitar) are adjusted to make sure that the string stays in tune on every fret, not just the open strings.
At least once a year, I take my guitars apart completely. I clean and polish all of the chrome on the bridge, saddles, tuners, and strap buttons. If it's a rosewood fretboard, I'll add some lemon oil as needed as I clean and degunk the fretboard. Then I polish the guitar body itself. At this stage I also check all of the electronics to make sure the solder joints look good and that everything is working properly.
So, where does that leave us? EVERY guitar would benefit from a proper setup at least once. After the initial setup, only minor adjustments (if any) will be needed for the life of the instrument. A guitar that is setup well will play better, sound better, and hold it's value better than an instrument that is neglected.
I offer basic setups for electric guitars for $20. I do all of the above (and more) and include a new set of strings. I'll also give you a guitar to use (if you don't have a spare at home) while I'm working on yours. Prices vary widely depending what shop you go to, but you can expect to pay on average $30-$50 + the cost of strings.
I will change your strings for $10 (including the cost of new strings) and clean the guitar and fretboard in the process. But, I would MUCH rather teach you to change your own strings. Bring a set of strings to a lesson and I'll take our normal 30 minute lesson and teach you how to change your own strings.
If you prefer to go to a guitar shop, I can recommend quite a few local shops that I trust to do a great job.
So, here's the bottom line:
1) Get your guitar set up and adjusted at least once.
2) Change your strings at least twice each year.