Pierre
Lots more advice on this thread
12 string guitars

Essentially you have 4 pairs of strings (technically termed 'courses') with the lowest 4 tuned in octaves and the highest 2 tuned in unison. So your normal 6th string, (that's the thickest one) is tuned to E as usual, but it is paired with a thinner string tuned an octave above it. To find how it should sound, go to the 4th (D) string on a normal guitar and play it at the second fret. That note is an octave above your open 6th string and your octave string on a 12 should be tuned to that note.

So then you have an octave A at the 5th pair/course (same note as the 3rd string 2nd fret on a normal guitar), an octave D at the 4th pair (same as 2nd string 3rd fret) and an octave G on the 3rd pair (same as 1st string 3rd fret)

Some 12 string guitars will tune to normal EADG(pairs one octave apart)BE(pairs in Unison), while some 'ring out' more tuned down to DGCFAD. With DGCFAD you would use a capo at the 2nd fret to get back to 'normal' tuning.

What I tend to do using a chromatic tuner is to do the 'normal' 6 strings first, and then tune up the octave strings. As you tune up the octave strings you'll notice that the extra tension on the neck causes the 'normal' strings to go a bit flat, so you'll need to return to these. If the strings are new you may then need to revisit the octave strings as they stretch.

Two extra tips
Be gentle when bringing the octave G up to tune. You can break a new string if you go hard at it. Not only is that annoying but the breaking string can lash your finger and be quite painful. I usually do this string last of all, and I often carry a spare octave G with me.

A twelve string needs a good capo (I have a couple of Shubbs), but you will almost always need to tweak the tuning a bit with the capo on. Typically the E and A (the non-octave 6th and 5th strings) will get pulled a bit sharp with a capo on, and you'll need to bring them down a fraction.

cheers
KP