If you really want disc brakes, it's new bike time. If you are replacing the forks, at least half the drivetrain and the wheels (you need disc brake hubs, and if you need to replace hubs then you really need to replace the wheels), then with installation costs you will likely be paying a very significant fraction of a decent new bike. Even a lowish level Altus/Acera/Alivio equipped bike will be better than the Haro just due to how more advanced components have become over the years. Even the lower level Shimano groups have hydraulic disc brakes now.
If you are willing to stay with rim brakes, then replacing the fork and some of the drive train can make sense. When replacing the fork there are two main things you need to deal with:
1) The steerer tube diameter, and whether it tapered and has larger lower bearings. Likely it has a 1 1/8" straight steerer tube.
2) The fork travel and axle to crown distance. The axle to crown distance is important to preserve the steering geometry of the frame, but does not have to be exact.
With the drivetrain you'll need to decide if you want to replace just the rear derailleur and shifter, both front and rear, or the whole thing including the cranks. Shimano Altus/Acera/Alivio grouos are 9 speed, and better groups are 10 speed or more so you'll be replacing the cassette and chain. 9, 10 and 11 speed cassettes will fit on the freehub body of your wheel.
If you go for a 9 speed group then you can probably retain the current crank, but you should determine if the bottom bracket is worn out and needs replacement. If you go for a 10 speed group the current crank really will not work.
