There are a lot of characteristics differentiating wargames from other game types, but one common one is the scenario-style wargame.
Usually tactical in scope, this kind of wargame presents a game system in its rules, while the actual contests are a series of "scenarios" that are generally presented separately. This sort of presentation is rarely seen in other kinds of games, which usually have just one basic situation.
And not all wargames are "scenario-style" games in the way I mean, even if they use the word scenarios to describe varying ways to play the game. In many games there are variations in the game presented, but these would often be better called "variants" than scenarios because they are essentially riffs off the same theme. Sometimes they are fairly minor variations that add a unit or two or subtract some based on some historical decision or event. Other times they are fairly major departures, such as starting a day earlier or with a completely different order of battle. Still, in most cases they don't change the game into something completely different, and mean that a lot of analysis and planning can be transferred as the player tries to determine what to do.
On the other hand, with scenario-style wargames every scenario is really, in effect, a different game. The order of battle, victory conditions and often even the map is unique to that scenario.
This can be a bit overwhelming, I think, especially for gamers who haven't played many wargames. The whole question of what-to-do can be hard to wrap your head around.
So you're sitting down for a scenario-based tactical wargame you've never seen before, you're still digesting the rules and you have to figure out what to do -- knowing that early missteps can be fatal. You have five minutes to scope out the game before your opponent starts shifting about in his seat.
Professionally trained military staff officers and commanders have years and training and experience to help them, but a casual wargamer needs some easy-to-remember and use framework for organizing his thoughts.
Here I think borrowing, in simplified form, the US Army technique of METT-T is helpful. It's basically five bullet points or checklist items that can help size up the situation in five minutes and help the player start executing a plan instead of just pushing troops forward and hoping for the best.
METT-T stands for Mission, Enemy, Terrain, Troops and Time Available.
Mission, of course, is the most important and yet still often forgotten. In game terms, Mission is usually expressed in the victory conditions. It does little good to slaughter enemy troops left and right if your mission is to escort a supply truck off the map and you let it get blown up. So the very first task is to determine what the mission is and what has to be done to accomplish it.
Enemy. Study the enemy's order of battle. A wargamer has much better information about his opponent than a real-life commander. Use it. Consider whether there are any gaps in capability you can exploit. Is the enemy relatively slow? Is the enemy poorly equipped in some way? Does he have a shortage of anti-tank weapons in a World War II setting or lack cavalry on a Napoleonic battlefield.
Terrain. Examine the terrain. Are there places where the enemy will find slow going or good defensive positions? Does the terrain create choke points where a small force can delay a larger one or is it wide open and put a premium of mobile forces?
Troops. Study your own forces. What capabilities does it have? Does it have special abilities that can be exploited or weaknesses to be avoided. In all cases the interrelationship between enemy forces and friendly forces is relative. Having a light tank available might be insignificant in a big armor battle, or the key to victory in a small firefight against a pure infantry force that has no supporting armor of its own.
Time Available. Time is never neutral. One side always benefits from things slowing down, one side always wants to speed things up. It's vital to understand whether time is on your side or not. Related to that is the question of initiative. Generally the tactical situation will dictate which side starts with the initiative, but losing the initiative when you had it or gaining it when you didn't will have a huge impact on who wins. Make sure you understand exactly how much time you have. A well-designed scenario will be set up to leave one move short of victory with average play.
All five aspects of METT-T are intertwined, which is why it's not a simple checklist. You can't simply think about the Mission and then forget about it while you consider the Terrain or the Time. Your mind needs to roll through all five over and over again as you analyze the scenario. Just because you have a large number of hoplites means that hoplites are your main strength, if the enemy has even more hoplites. It may turn out that your real advantage is that small number of slingers -- or the swamp covering half the battlefield -- or the fact that the enemy's objective is a long march away.
Most military planning tools are too specific, detailed or technically demanding to be of much use on the wargame battlefield. But I've found METT-T to be a useful way to frame my thoughts when I'm sitting down for an unfamiliar battle.