The first three places to look at when trying to reduce weight would be:
- Frame - but this probably isn't an option
- Fork - depends on what kind of riding and travel you're looking for
- Wheels - always an instant improvement but also depends on how much abuse you want them to withstand and such... includes (hubs, rims, tyres, discs as subcomponents)
Personally I'd start with #3 since it will probably get you the most gain in performance... unless of course your fork sucks horribly for what you're trying to do with the bike.