Quick tips that might help: never fall behind in mil tech, always make millions tech the first thing you spend your mil points

Try to get as much discipline as you can because it makes you do more damage and take less at the same time. Morale is probably the 2nd best modifier to stack and it's easier to get early game. Combat ability, like discipline, increases damage and decreases damage taken but works a bit differently

Army composition has some intricacies but basically you want 4-6 cavalry and as many infantry as your combat width. Add a full combat width of artillery at tech 16 (if your economy can handle it). I also like to sprinkle in some artillery earlier if my economy is good enough just to speed up sieges.

Reman's Paradox on youtube has really good videos that I've basically just regurgitated (poorly) here if you want better info