Strings

Choosing the right guitar strings is more important than you think. First of all you need to know that there are different strings for acoustic and electric guitars. They come in different gauges/thicknesses. 

If you are a beginner choose thin strings as they are much easier to play. If you find it hard to play your guitar you might want to consider buying thiner strings. If you're fingers are strong and you can play any guitar with any strings there is no reason why you shouldn't try thick strings which are good for playing heavier stuff. The thinnest you can get is probably 0.008 gauge, but I find them too thin. If you're only starting choose 0.009 or 0.010. Most rock guitarists would use 0.011s which aren't too thin or too thick. 

For playing classical stuff I recommend you use thick strings as they will definitely make the sound quality richer. For shredding and fast playing use thinner strings as they will allow you to move around the neck faster, because less pressure is needed to create sound. 

It's really up to you what strings you choose. You need to see for yourself and decide what suits you. It takes experience. Don't be afraid to try new brands or thicknesses. That's what they're there for! 

You should replace your strings often (I do it every month). With age your strings will begin to go out of tune very easily and might even break (I got hit in the eye once. not nice). Some strings will last longer than others depending on the brand, but they're also more expensive. Change your strings whenever you feel you need to.

You should also clean your strings regularly as dust may make it harder to play faster things. Clean strings are just smoother and nicer to play. To clean them use a soft cloth and just swipe over them and under them (if possible).