Take a look at the strings of your guitar. What sort of shape are they in? Are they discolored? Rusty? Are all six strings present and accounted for? If you answered no to any of these questions, or if it's been several months since you put new strings on your guitar, it's time for a string change. New strings make your guitar sound brighter, and generally make it easier to play.
How Often Should I Change My Strings?
Just like brake pads on a car, guitar strings wear out with use. Old guitar strings often behave badly - they'll lose tuning more quickly, sound less "bright", and give you problems with intonation. Old guitar strings also break, often during the most inopportune moment. Be sure to head into any live playing situation with new strings on your guitar, and several more sets of strings packed in your case, should you break a string during performance.
When I'm playing my acoustic guitar a lot, I'll change the strings at least every two weeks (more than that if I'm using it for gigs). It's probably not necessary for beginners to be quite as diligent with keeping new strings on their guitar, but changing strings a minimum of every couple months is a very good idea.
What Sort of Guitar Strings Should I Buy?
Everyone has an opinion on which strings are best, but let's put aside the discussion of guitar string manufacturers for a moment, and discuss the type of strings needed for your guitar. If you own an acoustic guitar, you need "acoustic guitar strings". If you own a classical guitar, you need "classical guitar strings" or "nylon strings". An electric guitar needs "electric guitar strings". And a bass guitar needs... wait for it... "bass guitar strings".
You also need to consider the gauge (thickness) of strings you'd like. This is where personal preference comes into play, but for beginners, I recommend starting with "medium" gauge strings, and varying from that as you develop a personal preference. An oversimplified rule of thumb is thicker strings provide better tone, but are harder to play.
If you're intimidated by the thought of buying a set of strings from a guitar store, don't be. Simply march in, and say "I'd like a set of XXXXX (brand name - eg. D'Addario, Fender, Dean Markley) medium gauge acoustic guitar strings please." Prices vary from store to store, but a set of acoustic strings shouldn't set you back more than $8 (several brands, like the excellent Elixir strings, cost more, but the merits of these products belongs in another article).