Could somebody give me a good discription how to tune the guitar(the tuning that Stills uses) , so I can play Suite : Judy blue eyes.

Hi Johnny, Stills usually uses "open D" tuning (D-A-D-Fis-A-D).
That means, you have to TUNE DOWN the two E-string TWO STEPS (two half tones) to get D.
TUNE DOWN the G-string one half - tone, you get a Fis - string.
Tune down the B-string 2 steps, to get an A- string.
This way you'll have 3 D-strings, 2 A-string and a Fis-string.
Do a continous alternating bass on the 6th and the 3rd D-stings with thumb, and play the melody on the 1st (now D) string.
Good luck from Budapest, Hungary !!!