Occasionaly you'll receive a report of a Task force from BdU. Intercept it, and there's a good chance you'll find a battleship or a carrier. It won't be easy because they move fast, and are heavily escorted.
Sometimes you'll find a battleship or a cruiser inside a convoy, going slow. Sitting ducks.
I didn't have much luck with large warships. During the years of playing, I sank no more that half a dozen battleships and not a single carrier, although I attacked and damaged a few.
Welcome aboard and good hunting!