how to write a case in sql ?

Question

select
      case a.[RoleName] = 'student' then 'its student'
      when a.[RoleName] = 'teacher' then 'its teacher'
      when a.[RoleName] = 'princi' then 'its principal'
      else 'attendant'
      end from  Roles]as a
inner join RoleDetails as b on
RoleId=b.RoleId
where b.Reserveid='18'

i am getting an error at 02,03,04 line at "=" 

error: incorrect syntax near =

 

whats wrong in this query if anyone can help me

thanks,

subin.