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Try playing 15k CWB if you're not playing that already. For CWB 15k, it's often good to get 6 gold/gold normal archers at least. Upgrade attack/defense attributes. In almost every occasion, they're better than experience.
There are 3 types of ways to base your army on. Rush, Skirmish, and Balanced armies. Rush armies contain no archers and all cavalry and infantry. Skirmish armies are mostly archer and cavalry armies. The aim here is to shoot the opponent down and avoid risky hand to hand fights. Balanced armies are... well... balanced. They balance the archer, infantry, and cavalry forces. You'll eventually learn what playing styles fit each faction best.
In a typical balanced v balanced battle, there will be 4 "stages". The initial archer war, the infantry fight, and the cavalry fight, and the cavalry flank charges. Of course, that's not saying you can't do something unorthodox! No two battles are the same, but those 4 stages often occur in most battles. You'll probably develop and learn some neat tricks later on.
In the archer fight, start shooting at the enemy missile units. Your aim here is to either win the archer fight, or at least knock the fight close. If you lose the archer fight, hopefully you depleted a good number of the enemy archers, since archers can be a huge threat throughout the battle. Don't use flaming arrows for the archer war. If you do win the archer war, feel free to direct them at the enemy infantry or cavalry.
For the infantry fight, simply engage the enemy infantry. If you have any excess infantry units, try to send them around the backs of the enemy infantry to flank them. Otherwise, you can use any extra infantry to aid in the cavalry fight.
About the same time that the infantry fight is happening, your cavalry will likely be engaged with the other cavalry units. Use wedge formation to maximize your cavalry's effectiveness. However, if your general happens to be in a cavalry unit, don't put your general in wedge. Putting your general in wedge puts your general in the front of the "arrow", a very unsafe spot. You don't want your general dying upon charge impact. Do your best to surround your opponent's cavalry. Often times, the player who surrounds the other cav force will be the one who wins!
Once you beat the enemy cavalry, you can either go after the enemy archers (only if your infantry doesn't need help), do a few flank charges, or split your cavalry force up to flank and chase down remaining archers.
If you're playing a 2v2 (or any team game), deploy next to your ally so you can support him or her.
Ask Max if you want more info on MP, since he's a lot more knowledgeable than me.
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CWB rules are the most common rule set. I'd use them, since most people know that they are.
In case you don't know CWB Rules, here they are:
Max 6 per unit
Max 8 Cavalry
Max 8 Archers
Max 2 Berserkers
Max 2 Horse/Chariot archers
No Artillery
No Elephants
Horse/Chariot archers count as cavalry, but not archers. This means that you can have a missile force like 6 normal archers, 2 lr archers, and 2 Horse archers. However, you can't have 8 melee cavalry and 2 Horse/chariot archers, because that breaks the rule of Max 8 cavalry.
Slingers count as archers.
Units such as head hurlers and javelin/pila armed ones don't count as archers.
There's also another rule set that is a slight modification to CWB. We call it RCC Rules and they are as follows:
Max 6 per unit
Max 8 Cavalry
Max 8 Archers
Max 2 Berserkers
Max 2 Horse/Chariot archers
Max 1 Elephant
No Artillery
Horse archers count as cavalry, but not archers. However, Chariot archers count as cavalry and archers. The beauty of this is that it weakens Egypt, which is way overpowered in 15k CWB Rules.
Slingers count as archers... as usual
Units such as head hurlers and javelin/pila armed ones don't count as archers... as usual
There's also a max 1 elephant, which counts as cavalry. It adds potential tactics that can be used in the game.