Exactly what JW said above. Then...
Siphon out about 10 gallons of water from your tank into a bucket. Dunk the rock in the bucket 10 or 12 times to get all the crap off it. With 100 lbs, you might need more than 10 gallons, but see how it goes. Lay all the rock out on a counter so you can see what each piece looks like. Give them a good look over... look for obvious corals, sponges, etc. Find the piece that you like most, or that have features you want to show off. Set those pieces aside and use the other rock first. Then place the 'good' pieces on top where they can be seen and admired by all who come to your home
Enjoy the rock... IMO, new rock is one of the coolest things you can get for a tank. I'll check out new rock twice a day and once a night for the first month or so I have it (looking for critters and such).