The guitar I'm using is pretty out of tune and I'm quite new to playing so I can't really tune by chords yet. I have a piano to use though but I'm not quite sure what I should do. I read the tuning lessons but I don't really understand the part with the "without resonating". Any tips?
"Without resonating" just means the notes are exactly the same...

For example, say you had one string tuned perfectly in tune and another string tuned slightly flat of perfect. If you were to play the same note you'd be able to hear the note "wobble".

Heres how I tune without a tuner... unfortunately you need atleast one string that is in tune.

Say your low E string is in tune...

Your open A string should be the EXACT note as the 5th fret of the low E. When its close you'll be able to tell... when you play them at the same time and you can't hear ANY difference in tone, it's in tune. Continue all the way down (using the 5th fret of the previous string as a reference point. The only time you don't use the 5th fret is when you're tuning the B string. For the B string you want to use the 4th fret of the G.

Hope you understand.
Can anyone tell me which piano notes corresponds which guitar strings(open).