I bought single player MB a long while ago and got invited to the beta so this is all from that time. In single player, get good at bow first to the point you can hit moving targets by predicting the trajectory. It's fun to be able to pick off a few of the horseman charging your hill. Then choose either shield or a two handed weapon. If you want to use a two handed weapon in multiplayer turn autoparry off and learn the directional parries. Good weapons to choose are Glaives, Large Axes, or the best 1h sword you can find. Don't charge into a group of people. If 5 are running at you, choose the nearest flank and try to get the guy on that side in between you and his friends. If you're fighting archers, carry a shield around even if you are using a two handed weapon, but even better than that is to stand behind a tree with a ranged weapon and pop out and back into cover with the right timing, preferably while no one is shooting at you.
If you are a mixed force commander fighting mostly infantry set your archers in a line on top of a hill, with your infantry slightly behind them. You should be on a horse with the cavalry following you, try to keep them together; it's more of a harassing force to spread them out or pick off stragglers,and more importantly you don't want to die. Order your infantry to advance ten paces once the enemy infantry threatens your archers, and they will form a line in front of them.
If you are infantry vs melee cavalry, you should have very good infantry or outclass the enemy somehow. Either way, these usually turn out to be the most epic battles for me for some reason. Find a rock or a tree, or even better a fence or something and basically order your men to stand as close together as possible. I usually have my cavalry stand in place directly behind the infantry to block charging horsemen. Also, against horses you should probably use something two handed with a lot of reach, like a glaive or two handed axe, or a spear even.
If you are up against horse archers, the best way to beat them is to hope you have advantageous terrain or to have your own horse archers/heavy knights. Infantry can sometimes outlast them until they run out of ammo and go on a final melee charge, but I guess it depends on a lot of factors.
So how does multiplayer MB work anyways? Is it like TF2 style matches how the beta was or is there a persistent world or something?