Choosing your long-term strategy
The first step after a game starts and you build houses, que up villagers etc. is to decide on your long term strategy. This decision should be arguably made by the time we start sending villagers on wood. The knowledge we have at this point is civ matchup and your map and these are the sole factors long term strategies are decided on.
This is where 80% of the new or even intermediate players go wrong. The usual mistakes are :
Deciding long-term strategy either too early
Some players decide the long term strategy even before seeing what civilization they are playing against and this decision is based solely on their civ choice. For example I have met numerous players who choose a civ like Franks and decide the strategy of going scouts into knights even before starting the game. Sometimes it may work for them but in cases where opponent has good counter civilization like Japanese or Goths and reacts well, they have a really hard time. This is usually not just because their build order was wrong or they made small mistakes but mostly because their long-term strategy was not decided correctly in the first place. Maybe the better long term strategy here was to go scout archer into throwing axe-men and cavaliers which could have been decided only after seeing the civ matchup and your map layout( resource placement, ease of walling etc. ).
Deciding long-term strategy too late
There are a lot of players in age of empires who either do not have a long term strategy or decide it too late. Even though this may seem like it helps you adapt to situations, it may not be the best idea. Having a long-term goal is important so that your small adaptations should be done in a way which leads to achieving your long term goal in the most efficient manner.
Let’s take the example of 1v1 in which a Japanese player and a Mongol Player are playing against a strong knight civ like Franks. The long term goal here for the Japanese player should be to go for maybe something like Halb-siege where as the long-term goal for Mongol player should be to go into Mangudai-Siege as Mongols don’t have halberdier upgrade available to them.
In Mongols vs Franks, lets assume both the players have an open map and decide to start scouts. We have a balanced situation in castle age where frank player decides to go 2 stable knights. In this case we need to decide our short term strategy on how to counter or defend it. We have few options: Pikes, monks, castle, your own knights, camels. In this case maybe going pikes isn’t the best idea castle with mangudai defense is the way to go as the upgrades for pikes do not help in your long-term goal of getting into mangudai-siege.
Similarly, In case of Japanese vs Franks, Pike-man defense may be good idea in castle age. because you want to get into halb-siege in the late game. Also, maybe an opening other than scouts like maa or drush could also be more viable.
Such decisions are possible only when you have a long term goal in mind. As said before, A pro player always plans ahead and maps out all possible situations which may effect his plan. This allows them to be prepared to adapt without panicking.