How do you fix a leaking sunroof in a 2001 brava?

All factory installed sunroofs that retract flush into the roof have a channel around their perimeter and there are drain tubes attached to the corners. The drain tubes run down the roof pillars and drain near the wheel wells. Typically sunroofs leak because these drain tubes have become clogged with debris, usually small leaves and stuff from parking under trees and leaving your sunroof open. If the sunroof can open wide enough to see the drain holes you might be able to press a hose into it that can shoot air pressure or water pressure through it to drive out the debris. If the car's in nice shape and your not mechanically inclined, you should take it to a local body shop and ask them to fix it. They can remove the sunroof for better access and do a better job.
If it's a cheapo after-market sunroof someone put in, it won't have the recess channels and drain tubes. They usually just have the glass pull down against a top gasket and you'll have to look for a spot where the gasket is gapping or damaged, then apply some silicone rubber.