In Empire: Total War, the Kingdom of Prussia is one of the nine playable major factions available for the Grand Campaign. Unlike other nations with nearly boundless territories such as Russia or the Ottomans, Prussia starts of with two regions, of which are critically split by Poland owned West Prussia: Brandenburg and East Prussia. Brandenburg's regional capital, Berlin, is also the capital city of Prussia, being Königsberg the regional capital of East Prussia. Both territories are moderately affluent, Brandenburg featuring a Trading Port, a College, a prosperous Craft Workshop and some profitable farms, and East Prussia featuring a Coaching Inn, an undeveloped farmland, and a Craft Workshop. Both regions have undeveloped towns that will become useful in the Late Game. Prussia's Land Army is already a powerful force, only matched by other great powers like Sweden; Prussia may have the upper hand in Land Battles, but at sea Prussia's power is shameful, not to say inexistent: Prussia, while starting with a port, doesn't have a navy at all, initially being forced to rely on Brigs and Sloops.
To succeed, Prussia must concentrate on its Land Army, forgetting the Navy at least initially. Prussia Line Infantry is historically the finest known in the 18th century and has Plug Bayonets right from the start. Prussia's Government may be the most important contribution to the army: King Frederick's "Obssesive" Trait, plus Prussia's great Minister of the Army, plus the Bonuses from being an Absolute Monarchy contribute to lower units' upkeep and recruitment costs some 30%.
After amassing a great force in both Berlin and Konigsberg, you have various options with varying levels of risk. Prussia starts with no allies, and choosing one may or may not be decisive: Hannover is the usual initial ally, for allying with them prevents backstabbing while expanding from Berlin; allying to Britain may upset France and Spain, but nothing to serious for the are miles away from Prussia; allying with Austria or Poland is a decision that is not the most advisable because it severely limits Prussia's possibilities of expansion.
Saxony is a good target for the Army of Berlin, adding an additional source of income, though making you closer to your main rival, Austria. Declaring war on Saxony places you against Poland-Lithuania, who then must be "pacified" by the army of Königsberg. Poland is easily destroyed if you have it as your only enemy, so avoid war with Austria. However as Prussia, you will eventually be fighting both Austria and Poland, do not bother to delay but strike first gaining strategic provinces. After destroying Poland, you have plenty of other options: expand north, destroying Sweden in the process; expand South, or finish destroying your rival, Austria, thus clearing a clear path to Italy; expand west, facing Austria and the Holy Roman Empire (The German Electorates and Kingdoms), and later France and the United Provinces as a result. India and the America are also good targets for the Prussian forces: Pirate holdings at the Caribbeans or Dutch owned Ceylon (modern Sri Lanka) make up for a great base of operations that will certainly result in a prosperous Prussian Empire,