It would be good to have a replacement tank ready but first it's best to find out where it's leaking from, as water could be coming from an overflow or even a filter, which just makes it seem like the tank is leaking.
If the tank is leaking the best thing to do is get a new tank and then drain the water from the old tank into buckets. Move all the corals, fish, LR and inverts into the buckets, make sure you have a power head or air stone running in the bucket to supply oxygen. At this point switch the tanks over and move the LR and sand, if you have any, back into the new tank and add the water back in. You also want to make up some fresh saltwater that you may need for a water change or addition water. Turn on the filter/pump and let everything run before you move the livestock back into the tank. You may get a small cycle, so the water change will help with that.
