Know Your Enemy
"We have a very daring and skillful opponent against us, and, may I
say across the havoc of war, a great general."
Much of coming up with a successful strategy depends on having information about the enemies abilities. If your only knowledge about your opponent's army is that they're painted blue, you aren't going to have much hope of figuring out what they're capable of.
Information can be gained in a number of ways. You can buy all the codexes, and read them all. You can read various forums around the internet. You can ask your opponent what their units are and what they'recapableof.
However you plan to get this information, you do have to get it, or you won't be able to make an effective strategy. While the rulebook says an opponent doesn't have to tell you what is in their army, in a friendly game, ask anyway. And worst-case, ask what the stats on units are as your opponent is deploying them.
As your opponent starts placing their models on the table, ask questions about them. If they're vehicles, ask questions like, "how far can that move", "what does it shoot", and most importantly, "what kind of armour does it have?" If you don't know this as you set up your models, you cannot create your plan for how to address these vehicles.
As they place their troops, ask the same questions. How far can they move, what kind of armour do they have.
And as they tell you what they have, you start making mental notes, matching up the units your opponent is deploying with the units that you have that will be asked to handle those units.