The most common tuning for slide is D major.
The notes are ( in descending order ) D A F# D A D
To retune a guitar from standard tuning do the following;
Tune down top E string to the D note found on the second string third fret
Tune second string down so that the note at the fifth fret is the same as the retuned top string.
Tune third string down so that the note at the third fret is the same as the retuned second string.
Fourth string stays as it is on D.
Fifth string stays as it is on A.
Tune sixth string down so that the note at the seventh fret is the same as the open fifth string.
Finally pick through the open strings to hear what should be a nice open chord of D major and tweak any notes that don't sound quite in tune.