The build depends on two things, what horse do you want to ride, and do you want shield and/or 1h with the build.
Sounds like you've settled on the courser (good horse for not too much money).
I was always fond of the 18/18 build for courser (at level 30) but that's because I always had 1h/shield in my build. If you are going pure polearm, no 1h/shield then I'd suggest going 21/18. If using 1h/shield, then go 18/18. Here's my build suggestions for both (you can test the WPF before saving your character stats):
18/18
1 IF
6 PS
4 Shield
6 Athletics
6 Riding
6 WM
Split WPF between 1h and polearm (I usually did an even split, or would favor 1h slightly)
You don't need more than 4 shield on this build.
21/18
7 PS
5 Athletics
6 Riding
5 WM
All points into Polearm WPF (you may even want to drop to 4 or 3 WM and have 6 athletics and maybe some IF, I'd experiment with the WPF at the different WM levels before finalizing).
My favorite cav builds are (depending on horse you want to ride, i.e. pick the lowest agility you can to get by with the horse you want):
1h/shield/polearm: 21/15, 18/18, 15/21
Pure polearm (or 2h) cav: 24/15, 21/18, 18/21
