OK, a more serious answer . . .

For men, there is a WIDE range of possible wearing, from t-shirts to tux levels.
For women, it is even greater.
The main thing is to coordinate with the tartan of the kilt (assuming IS a tartan).
DON'T wear a gray fleece pullover under a Prince Charlie jacket with burgundy hose and bright red flashes with a kilt with no red at all. (seen it in Hendersonville, NC-> Barclay took pics!)

With a touch of thought, it is not hard. A killt just takes the place of both pants and shorts (and for females, regular skirts). Dress as appropraite normally, with just the necessary alterations to "fit well" with the kilt. To me, shaping and avoiding sloveness is more important than length (THOUGH, those often go hand-in-hand, such as with jackets (with the focus on how the front fits around the sporran.

IF you want more specific advise, please provide more info and direction as to tastes, intent, and level of formality.
It is not too often women ask men for fashion advise . . .

As for under the kilt, that is for you and your man to figure out.