There are several factors when deciding on which strings may be best for your instrument. The acoustic guitar, and how you intend to use the guitar has a lot to do with your choice of strings. Classical guitars differ from regular acoustic guitars. If you are uncertain on what type of guitar you have or what are the proper types of strings for your specific guitar, it is best to take your guitar to a reputable music store and have a professional look at it and ask you some important questions. The professional will be able to help you make a good descision on which strings based on your instrument, your playing style and your intentions for what you desire to play on your instrument.
Electric guitars are not so critical as acoustic guitars. Basically you'll have to decide what style of music you will be playing, what type of playing you will be doing, lead, rhythm, both. If you need help deciding, take your guitar to a reputable music store and have a conversation on what types of strings would be best suited for your genre of music and your playing style.
Bass guitars also have a wide range of options when it comes to strings. If you are new to playing a bass guitar or regular guitar and just don't know what would be best for you or even what types of strings are currently on your instrument, again, take your instrument to a reputable music store and have a professional assess what types of strings you currently have. They should ask you several questions to help you decide what suites you best.
Once you have a set of strings that you like on your instrument, always save the package the strings came in. Then when it is time to replace your strings you will know exactly the brand and gauge of your strings. If you change gauges of strings, you may need to adjust the intonation of your instrument to play in tune. To learn more about intonation please click here.