Question 1: Does your shotgun have a removable choke tube? If if doesn't, the barrel will be coded to indicate the fixed choke's constriction. Most removable choke tubes (they have a special wrench for removal) are marked with the constriction.
Question 2: What do you intend to do with your gun? Choke selection depends on anticipated application.
If you give us your gun's make and model, it will greatly help.