good advice here. Are you riding with cycling specific shoes? (ie do you clip in?) If not - get cleats and pedals - you generate more power that way. Yes - sit down as much as you can and breathe deep and relax. Pull up on your handlebars (you should be on your tops) Stand up only when you have to. You can also "paperboy" to reduce the perceived steepness of the incline. Rather than ride straight up a hill - loop from side to side. (don't do this in traffic!!) Don't worry - you will get better!
"Character cannot be developed in ease and quiet - only through experience of trial and suffering can the soul be strengthened, ambition inspired and success achieved" - Helen Keller