When using a 1 hander and shield:
1) Hold that block until they hit your shield, this stops feints and such killing you. But don't overdo it or they'll just keep attacking before you can get a chance to hit back. If you find yourself being left swing spammed then move to your right and back while looking at the enemy, and then hit them back with an overhead or left swing yourself.
2) A good tactic is to feint or hold an attack(Only a second longer than a normal swing). Double attacking is fairly risky but it's good if you know the enemy uses those two things a lot. Chambering is only something that should be attempted if you are very good at warband, so I won't tell you much about it
3) In mob fights, you generally want to keep yourself in a space in which you can quickly manueover in. This way, if you see someone with their back to you, just forward and slash them, then you HAVE to block and go on the defensive again, because the enemy will see you being aggresive and try to attack you. I often see people make the mistake of being over aggresive.
4) Pick a 1 hander for the occasion. Swords are good in the first rounds because people all have tunics etc. Then it is generally wiser to buy an axe or mace if you can, due to most people having armour or better shields.
If using a spear however:
1) Medium spears are the best spear for versatility. You can do fairly well in a 1v1 with it and it's still good at poking people as a support weapon. Short spears are for 1v1, and terrible for support really, too short. Long spears are great at supporting, but you cannot block(unless using svia spear from vikingrs)
2) If you're being bogged down by an attacking 1hander and shielder, walk backwards in a straight line so they are walking straight at you. Try to kick them as they come to facehug you, then get a quick thrust in. This will usually make them keep their distance which allows you to hit them with your spear.
2 handers require much more skill to use. I would suggest you stick with the above tactics
Archers and Cavalry are much more of a support role in this game. Archers should aim to take down the enemy cavalry if they can, but otherwise shoot into open gaps to put pressure on the enemy. Cavalry should try to knock down the enemy infantry but also keep an eye out for a chance to kill the archers.
Oh and save your throwing weapons for after the fight, they generally never get a kill at the start of the fight. Sure you might die and lose the chance but is it really worth throwing a javelin into an enemy shield or worse hitting the ground and giving it to them?
And stick with your team! I cannot stress how it helps to stay with your team.