Spear on horse is pretty hard, since you need good aim and exact timing. I started out using 2H weapons on horseback since they have a wide reach and are more forgiving on timing due to that. That gave me some confidence (I used this setup in campaign so got a ton of practice with it). With the nappy edition I find myself being able to cut down infantry pretty well with the shorter blades (ride close and let the blade be in slicing motion as you pass).
Best thing to to do is Host a multi game, put in some bots (not to many, 8 or so per team is nice, even less at first) pick cav and play a few rounds. The bots will often be distracted (archers in particular) so you will get plenty of chances to strike directly at them. Live humans are much harder (they aim for your horses head and use spears if they have them thus outranging you), but practice against those bots will be golden down the road. I use Arena map becouse its nice and tight and pretty much none stop action.
On horse you can dodge bots (cavalry wont really chase you), so you can target those archers for easier kills while you get the hang of it.