How do I work out my Course Handicap for the tees I'm playing?

For an 18-hole round (Rule 6.1a):

Course Handicap = Handicap Index × (Slope Rating ÷ 113) + (Course Rating − par)

The Slope Rating, Course Rating and par are for the set of tees you're playing. The result is rounded to the nearest whole number for use; for example, a Handicap Index of 12.4 on tees rated 71.8 / 128 with par 72 gives 12.4 × 1.133 + (−0.2) = 13.85, so a Course Handicap of 14.

Your Course Handicap is the number of strokes you receive from those tees before any format allowance. Your league then applies its handicap allowance to get your Playing Handicap.