Everytime you upgrade your unit building buildings, you get a better troop.

Never use peasants in combat, and the same goes for the militia or town watch (unless you are average-expert at the game).

It takes practice to use the javalin throwers - seen as light infantry - missile usually.
So use normal infantry instead of them till you are used to them. If you are playing Roman then that would be the Hastati. But not that they also can use pila (another kind of javalin) so that makes up for it.

Again if you are playing Roman - advance your archery buildings to produce archers asap, as they are alot more useful than javs...

If you dont already know - dont charge you cavalry head on with most infantry. Try to hit them in the flanks (side) or in the rear. Pin the enemy with you infantry before doing this to maximize the effect.

There are many areas of this forum with great tips for the beginner, have a browse cos its still quite useful for even good - great players.