How often should you replace your strings? Well, that partly depends on how often you can afford to change them and how much spare time you have. Given the cash and the time, I would suggest that you change the strings every 100 hours of playing time (That's more than 3 hours every day for a month). Pro's would change them every day or every week at least, but for casual players, once every 5 or 6 months is reasonable. When your strings lose their tone or begin turning black in some places, or you find that notes in some frets are in tune while notes in others are not, then it's definitely time to change your strings.
Strings come in sets defined by the range of gauges for each string from the top (thickest) to the bottom (thinnest) - or vice versa. They are made from various materials, including nickel, steel, brass, or nylon, and vary in price according to the materials, the process used to make them, and of course, the manufacturers name. Some sets come with extra strings, particularly the 2 thinnest strings B and E (or the 5th and 6th). You can even buy individual strings at some guitar shops, which allows you to pick and choose your preferred gauge for each individual string. Of course, in that case they do cost more. Light gauge strings are easier to bend and they allow you to lower the bridge more so your strings will be closer to the fretboard (making it easier to play), but, they do wear faster and lack the tonal quality of heavier gauged strings.
There are basically two types of metal strings: flat-wound and grilled. Flat-wound strings have a smooth surface and don't produce as much "scratch" noise as you slide from one fret to another. Their tone is not usually as bright as grilled strings and they are more popular with jazz and classical guitarists. Grilled strings have a brighter sound and tend to give a "punchier" tone. They are often the choice for acoustic guitars, but you will probably see them more on electric guitars, particularly when the music is rock, pop, heavy metal, and punk. They do cause a "scratching" noise when you slide your fingers along them, but, some guitarist find that appealing and use it as an added effect.
If you can afford to buy new strings often or buy two sets at a time simply unwind your strings and discard them. If you can't afford that, then remove all of the strings from your guitar and inspect them carefully. If they are all in very poor condition discard them, but if a few appear to still have some life left in them, you might want to keep them as extras for emergencies. Every time you replace your stings, discard the older "standbys" and save the ones you have just removed.
You don't want your new strings to slip or jam after you install them so be careful how you wind them onto the tuning spindles. Insert the strings into the holes near the bridge. Each guitar manufacturer has a slightly different setup for locking the ring or stub at one end of the strings, so make sure they're inserted correctly and completely. Pull the string all the way through and thread the loose end through the tuning spindle slot or hole (whichever you have) and pull it through a few inches. Bend the string at the spindle TOWARD THE OPPOSITE DIRECTION TO WHICH THE SPINDLE IS TURNING WHEN YOU BEGIN TO TIGHTEN THE STRING. Hold the string there and take up the slack with your other hand. The string should not slip out of the spindle. If it does, you need to re-bend it. Push the string another half inch through the spindle hole and bend it again, making sure it is in the OPPOSIITE direction to the turning of the spindle as it tightens the string. Keep constant tension on the string as you wind it tighter it and remove all of the slack. Allow the string to overlap the bend you made on the first turn and continue to wind the string while keeping tension on it with the other hand. Tighten the string slightly more than what it will be when it's tuned (you might need an electronic tuner to judge that), but, be sure the string is not loose. Use the same method to install all six strings.
If your guitar has string guys (as seen in Figure GM-2) for one or more of the strings (little guides near the tuning spindles) make sure the strings are below them before the strings are tightened.
After all six strings have been installed, tune the guitar, but do not cut off the excess string. Leave it overnight and retune the guitar the next day, if necessary, then cut off any excess string. (You can read more about GUITAR TUNING)