All I'll add to bikeman's reply is that if you don't have a big enough adjustable wrench and if you don't mind some small marks on the big head set nuts that you can use a pipe wrench or water pump pliers as well.

Also your present fork and headset are what is called "threaded". That means the top of the fork's steer tube is threaded. Your replacement fork would also need to be a threaded type and the steer tube must be the exact same length to within a 1/16 inch at the outside to fit your frame. If the threaded steer tube is only slightly longer you can cut it down. But only if the upper threaded bearing cup does not try to screw down onto the part where the threads are not fully formed. If the threaded steer tube is too short there is no way to use it. If the fork has no threads and appears to be much too long a steer tube that means it is one of the newer style threadless headset styles. That's fine but it means you would need to buy a threadless headset to use that fork and you would need to buy a threadless stem and some spacers. All in all it begins to take the repair out of the budget range in a hurry if the fork is not an easy drop in replacement.

Anyhow you have a bunch of options from replacement of the fork to "locking" the old one. Size it all up and select your poison... And best of luck whichever way you opt to go.