There is not easy answer for unit choice.
But, at first tiers (I - III) you should make self-sufficient armies, because most new players will not help or strategise with you.

This means:

• If you have archers, have someone to protect them, or be ready to run.
  
• If you have artillery (Romans have it on tier III), make sure you have someone to protect them against enemy troops.

If you know what type of unit you want to play, you can forget the diversity bonus and play with 3 of the needed previous units, this will make progress much faster.

First artillery has medium range and will not be as devastating as you might think, but it can build defences, which are hard to spot and play against for new players.

Friendly fire
Frendly fire is full in this game, as a result you can damage your allies more than your enemies, make sure to NOT fire at engaged enemies.

Hiding
Forests, swamps and high grass are excellent at hiding troops. You need to walk right next to them in order to detect, thus making these locations key for easy ambushes.

There are mainly 2 ways of playing:

• Defend
  
• Attack

Defend
This means staying back and taking good positions (highgrounds, blocking passageways etc.) and waiting for the enemy team.

Pros:

• Easier to execute
  
• If done properly and with some help from your team, very effective

Cons:

• Can be overrun by enemy forces
  
• Drags the game out

Attack
This is the opposite of defending and means marching forward with intent to capture the enemy base.

Pros:

• Qucker to get to the "fun" fighting part of the game
  
• You get to participate in the bigger fights

Cons:

• Harder to properly execute

Alternative attack
Some units (ranged, some melee infantry) have an alternative attack (melee attack for ranged, ranged attack for melee) which is ordered by pressing down ALT. If the cursor changes, that means the order will also change.

Cliffsides
Practically all cliffsides in the game can be held by ranged units, giving larger area and good ranged defence. In order to hold a cliffside hover the cursor over the top cliffside edge, while having units selected, if you see a small circle next to the cursor and lots of arrows next to the cliffside then ordering units will make them hold the cliffside in a thin line.

Focus fire
Any tier II or above ranged unit has Focus FIre ability, which allows them to target a small circle. The ability is only available if the units is NOT attacking anything and can be used to attack enemy even without detecting it.

Friendly fire
If you feel comforable with game controls you can try to fire at engaged enemy units, as long as enemy untis are between your ranged and your ally units. This will not fully prevent friendly fire, but it does more damage to the enemy than your ally. DOES NOT WORK FOR ARTY