I was given a sweet early 90s cannondale 3.0 mtb. and would like to know what measurements i need to know to go to a suspension fork and how to take these measurements.thanxs

You will need to know if it has a 1" diameter head tube or 1-1/8" . The smaller size was more common in the older designs, but the larger did exist and is possible. The next dimension is the steerer tube length. You will probbably need a new headset and handlebar stem to go to threadless(if you already have threadless, cool).

The steerer will usually come long if you buy new, so measure well before cutting. You want to put it all together(with any spacers for options), with the headset, stem and spacers and give yourself a little margin for the first cut. Easier to remove than to add as the saying goes

Actually if its Cannondale, its probably a 1 1/4" headtube or (you're screwed then) or 1.5" (not as screwed but still probably not worth the cost).

Did they use 1-1/4" on non-headshock models? I do recall there is a spacer to adapt a standard 1-1/8" headset into the larger however.

CF fork I suggest.

Oh fudge...backwards conversion.
I do have a stem for Cannnondale thick steerer.
P.M and I'll check if needed.
Long one, pay it foreward if needed.