Usually, the letter of recommendation is directly sent to the school/employer, so you have to tell anyway to the professor.
If you don't have anything specific and are taking one for the future, you can tell him so.
No there are no formalities involved other than asking. But, please take recommendation only from someone whom you are confident would give you a good recommendation. Bad recommendations reflect very poorly on you and can most often be the reason for rejection from school/employment.