Note: Yes, a lot of this is copied and pasted from guide for Bulgaria. Their starting position in High is very similar, so similar strategies apply. But they have different units, so if you like the location but not their units, choose Bulgaria instead.
Serbia - High
Serbia in High is an interesting faction to play, more than early or late. Serbia has a central location on the map, in the middle of everything. It's far enough away from western europe to avoid becoming embroiled in the constant warfare there, and far enough from the conflicts in Asia Minor. And you are just far enough away from the east that when the Mongols come, they will help rather than hinder you, as they draw troops away from you.
The enemies around you are not very dangerous... the Hungarians are the major threat, but they tend to be distracted between the HRE in the west and the Cumans and Polish on the east. The Bulgarians are small like you and, while you should keep an eye on them, shouldn't cause too much trouble. In short, you are a forgotten corner of the world. Forgotten, but not for long.
Nearby as well are two very rich provinces - Greece and Constantinople - which are ripe for the plucking and will give you the finances needed to proceed with your world domination plans. The Serbians have some of the best cavalry in the game, along with the normal, decent halberdiers, along with decent bow units. Serbia and Bulgaria are very similar in location and starting strategy, so which you use really depends on whether you prefer excellent cavalry, or infantry armies. Serbia is probably slightly more difficult, given that it is slightly less insulated from Europe and is on the Adriatic instead of the Black sea, but the difference is minor.
GETTING STARTED
You start out with not a whole lot of troops, and none of your good ones. But there's no point in trying to rush anyways; no one around poses much of a threat. Your first order of business should be securing the rebel remains of the Byzantine empire, Greece and Constantinople. Both offer rich provinces, both in farming and in commercial possibilities. Constantinople also offers much more than that.. taking it gives you a whole slew of titles to confer, as well as a fortress and instant access to wargalleys. How you get there is up to you. But since you need money anyways, my usual order of business is to build... an emissary. That's right, we're going to do this with bribery. At least you want to bribe Greece... the general there, Leo Sgouros, is definitely worth making use of. He's got 4 command and something like 6 quills of acumen. Save him for Constantinople; you can give him the governorship of Constantinople, and them either the Patriachate (for a 6 star general with something like 8 quills) or the Captain of the Stables (for an 8 star general with fewer quills).
Bribing Greece is not something you can do right away. You might want to build a couple units of Halberdiers for security at first, and then forget about units for a while. Sorry, Serbia's infantry is decent in early, but by high, regular halberdiers are better. But we need money. If you try to bribe Greece on the second turn, they will want 7000+, which you don't have. You can build a few buildings, you should build a church at least, but don't go overboard.
If you keep trying every turn, some turn Greece will drop to ~3500 florins and greece will be yours (Constantinople will ask for ~11k florins). You'll need to move some of your troops into greece for pacification, though you can try to spawn some rebellions if you're so inclined as well and get some money that way. Build money buildings in greece - forest clearing, etc. You should be making some money now, ~700 a turn or so. Each turn try to bribe constantinople. Let crusades through if they come; they will soften the big C for you. Each turn they seem to want less, so eventually they will give you what you want.
Now you've got 3 provinces and a good base for a trading empire... or any other kind of empire you want to build. You should be pulling down ~2000 florins/turn now, enough to do just about anything you want. And so far, you haven't even had a battle. You have all kinds of options... start conquering immediately any direction you want. Build a trading empire and go for strategic conquests around the map. Reunite the Byzantine empire under the Serbian banner. Or various turtling methods... wait until the Mongols arrive and snipe at the Cumans back, moving into the steppes, or the Byzantines, into Asia Minor. Wait until Hungary gets involved in wars with Poland and Germany. The options are pretty open here.
SPECIAL UNITS
Voynuk Bladesmen - 'Almost' billmen. In high, these really don't pack enough of a punch, though they have cheaper upkeep than halberdiers. Skip them.
Vlastela Heavy Cavalry - These pack quite a punch, let me tell you. They have high build requirements, needing an Armorer's Workshop, Horse Breeder's Guild, and Spearmaker's Guild, but they are even better than the High didebuls, better than Chivalric Knights, so one of the better cavalry units in the came. Only things like Lancers and Teutonic Knights are better. Make good use of these.
Carska Garda - These are your royal cavalry units. Like the Vlastela Cavalry, except smaller units with lower upkeep. You should get the other ones instead. There are early and high carska garda, like the Armenians, but since you're in high, you don't need to worry about that.
Serbian Cavalry - Your resident Horse Archer replacement. Not as fast, but heavier duty. Similar to Bulgarian Cavalry.
Voynuk Swordsmen - A decent and dependable, but basic, sword unit requiring only a swordsmith. More or less identical to Feudal Men-at-arms. The best sword unit the serbs get; lean on the bladesmen or halberdiers instead.
Shielded Bowmen - Decent bow units, and not bad in melee either. Even with arbalesters, you should find a use for these.
Strange fact: Serbs can build Jinetes, but Bulgarians can't. Also, Serbs get the Barque-Carrack line of ships, while Bulgarians get the Dromon-Gungalley line of ships.
