Assuming you don't have a key, It depends on how old the lock is. If it's a mortice type key for a Victorian or Edwardian lock, you may be lucky and have a local locksmith who can cut a new one from the existing lock. It will need removing and taking it to them. Otherwise we can cut/make keys for most antique locks.

If it's an older antique lock with a bridge or Georgian/skeleton type key, not easy, there are some specialists (incl us) that might be able to help.

If it's a mortice type key then a decent High St locksmith should be able to copy it, they may have to order the blank. The older style keys, again not easy. Some size blanks are still available and you might be lucky that your local High St Locksmith might give it a go, otherwise contact a specialist. 

We do refurbish old locks for customers incl doing the keys where possible, check our restoration page, we try and keep it updated.

There're a few terms for pre mortice keys 'warded', 'bow', 'Georgian', 'bridged'.