You should clean your guitar each time after you play it. The oils and sweat from your hands get deposited onto the strings and wood, causing corrosion of the strings and damage to the glossy finish on the wood. A soft cloth should be used to wipe down the wood (neck and body), and an alchohol swab can be used to wipe the strings (do NOT get the alchohol on the body!). Guitar stores sell special cloth and wipes for this purpose, it is best to get a suitable product that you know will work well on your guitar. This may seem obvious, but: NEVER wash your guitar!

You will eventually have to change the strings on your guitar(s). The life of strings depends on how much you play and how you play. Whenever you hear the sound of your guitar starting to go dull, or if it constantly goes out of tune, you may need new strings. Always change all the strings at the same time - an exception could be if you break one string soon after a string change, then change only that string. When you are changing all the strings, you have the opportunity to do some more cleaning and maintenance. You CAN remove all the strings at one time - but you must get the new strings back on promptly (don't put it off for another day). If your acoustic-electric guitar has an active pickup that uses batteries that fit through the soundhole, change the battery while you have the strings off. But mainly you have the opportunity to clean the fretboard - use a damp paper towel folded flat and gently wipe the frets and the fretboard between the frets. Do NOT drip any water on the fretboard, just a slightly damn paper towel is all that is needed. Immediately wipe it off with a dry paper towel, ensuring no water is left anywhere.