Okay there a lot of guides and tips out there for a lot things, but I have yet to seen one for PVP. so here a few tips for you who want to play PVP but don't know what do or just need some minor tips.
First things first, expect to lose a lot in the beginning. just because it seems you never win doesn't mean you are bad player, you just need to find the thing that works for you. so get out there and experiment a little bit and try different units mixtures and tactics.
With that being said, remember to play to your racial strengths. Elves should find open ground for their archers to do some serious damaged to the enemy, dwarfs should try to use their heavies in spots were the enemy can't flank them, and the undead should bring in max numbers to overwhelm the enemy. And find a Experience players for your race to see if he got any helpful tips on how to use your units.
Don't just rush in. This is very simple ground rule, remember this isn't AI controlled army, this a human and he probably knows how to deal with a charging enemy. also more tips on this, if the your opponent has archers remember to use cover if you have to advance. See what your fighting, if they have better units, but less troops try to move your units in way to surround and flank their units, and if you have the better troops try to force the enemy to fight you in frontal assault.
try to keep your units together. I don't mean form the bubble, but try to keep your men together so they could work to their fullest and support each other.
If you have teammate(s) in match work with them. come up with a plan if you can. Also this has happen to me more often then I like so here something you never do, charge the enemy ( again this is important) and leave your teammate behind to wonder what the heck your planning. The chances are your are either charging into a much larger force or one that would dispose of your units like flies and go to finish off your teammate.