Kilts historically originate from Ireland and moved over to Scotland through immigration.
There are no set rules as such but there are a few guidelines;
A true kilt must be of a approved Tartan (the pattern), Tartan's denote what clan you come from and so are very important.
For men kilts are generally knee high, although for women it may be slightly shorter.
Usually those wearing traditional kilts at formal events (such as weddings) will have knee-high socks called Hose's.
Kilts usually have what is called a Sporran (pouch) on the front, this acts as the pocket of the kilt.
Traditionally those wearing kilts on formal occasions also have a small knife called a Sgian Dubh, which in modern times is more decorative than defensive.
Unlike your picture the appropriate top half is a formal shirt or blouse ;).
No mention of underwear - but when Kilts were standard dress there was no affordable equivalent, so they did without! I suppose in the modern day this would depend on the occasion and how, erm, proud the particular individual was ;)