Short answer: Watch an LP, read guides and play another 50 hours (and maybe you'll start knowing what the hell you're doing.)
Long answer: "what you're doing" changes heavily based on the nation you're choosing but there are several things that you'll need for all of them.
1) Expansion
Arguably the most important thing in the game, expanding against independent enemies is extremely important. The more you expand, the more income you get, the more possible gem income sites you will find.
Obviously an early-game phase (usually first twelve turns depending on map size). Be aware that if you manage to get TOO big there's a chance people will see you as a threat and dogpile you.
2) Spend your gold
If you end a turn and you have more than 100 gold (and you're not saving up to build a fort) you're probably doing something wrong. Always recruit mages whenever possible, and troops.
3) Build all the forts
Forts boost your income and allow you to build more commanders/mages and troops. It's usually not a good idea to build forts between two and three provinces away from each other so they won't steal too much of each others resources.
4) Armies win early game, mages win late game
What it says on the tin. Early game, if you have more troops of higher quality, you win.
Late game you have spells that can allow a single mage to tear down an entire army on his own, or thugs that can fuck your shit up.
In the late game, armies are used as buffers between your mages and the enemy so he concentrates on your army instead of your mages. (or if you have things like the super buffs such as mass flight, weapons of sharpness, armies of gold/lead etc).
Also, use your gems.
In regards to pretender creation:
Is your nation a bless nation or not?
A bless nation has some really good sacred troops (more than one attack, high protection and HP) that would benefit greatly from a bless (usually a major bless such as water 9, nature 9). Chances are that if it is, you'll want a pretender that gives them said buffs to make them unstoppable killing machines in the early and mid-game and still a formidable force later.
If it isn't a bless nation (your sacreds aren't worth investing in because they are way too expensive to get a decent amount of, or just have shit stats) you'll probably want to go for scales.
If your units cost a lot of resources, get production 3. If they barely cost any, get sloth 3.
Order 3 and Growth 3 is the best way to make sure you get good income, and Growth scales also makes your old mages less likely to get diseased from being old.
Heat/Cold you choose depending on your nations favourite and your opponents. If you're playing against mostly nations like Agartha, Abysia or Xibalba, take cold. Those nations suffer because troops are either cold-blooded and suffer penalties in cold dominion or focus on fire that gets penalized in cold.
Luck and Magic: Never ever take Misfortune 3 and Drain 3 or you'll have some very very shitty events knockin' at your door. Order 3 and Luck 3 don't work VERY well together, and are very expensive. Some people go for Turmoil 3 and Luck 3 for more events. Magic buffs research rate in your dominion so always try to get it.
Other than scales, your pretender can be awake/dormant/imprisoned. Your default should be a dormant pretender because it gives you much more leeway. Imprisoned you shouldn't take unless you have something very specific in mind (because having him up at turn 36 is pretty late), not to mention if you get rushed early game you may not have anything else to threaten your opponents with.
Awake pretenders are USUALLY combatants, pretenders that can expand into provinces by themselves (never on first turn because you can't see what's around you and you may bite off more than you can chew (heavy cavalry/cataphractst WILL fuck you up. Lizardmen will curse your ass and possibly horror mark you.) Usually taken for nations that don't have great troops. Examples of chasses can be either of the three dragons, the Gorgon, Virtue and the Cyclops.
Arguably the most important thing:
Plan your research goals ahead of time. Know what your national mages are capable of, what spell boosters (items that increase effective level of magic paths such as Earth Boots, Skull Staff, Fire Helmet) they can make and how far up the spell paths they can go. End game is all about magic, and if your magic is shit, you can't expect to ever win. You can build your pretender to be able to forge boosters that your nation can't. There are several powerful spells that people usually aim for (flaming arrows, rain of stones, thunder strike, fireball, horde of