Should I Believe That I Receive Healing or Command Sickness to Leave?

Either method works.  The first is based on Mark 11:24 and the second is based on Mark 11:23.  In fact, there are many other ways to get healed, too!  Take your pick!

If you are ministering to someone ELSE, you would command sickness to leave, because you can’t do someone else’s “believing he receives when he prays” for him.  However, you can encourage the other person to believe that he receives when you lay hands on him.  Technically, you could just lay hands on the person in obedience to Mark 16:18 without speaking to the sickness, as laying hands in faith is all that is required to fulfill your end of the bargain in that verse.  However, you would want that person to give you permission to do that and to have some understanding of what you are doing.

See also:

If I Start to Feel Sick, Should I Receive Healing or Enforce the Fact That I’m Already Healed?