Definitely use a tuner. You'll have to tune by ear if it's acoustic but just make sure the low E(6th string) is in tune, then you can proceed to tune the rest of the strings to each other. Example hold down the 5th fret on the low E string and play it and the A(5th string). You'll know it's in tune once both sound exactly the same. You can hear the frequencies beating when it's out of tune and not beating when it's in tune. It's easier to show you than to explain but I'm betting there's a youtube video of it somewhere.