With the navy in port in the same province as the army, select the army, select "embark on ships" (or something like that) and then move the navy wherever you need it.
Alternatively, with the navy in a sea zone adjacent to the province the army is in, give the army a move order into the sea zone (bear in mind this is slower)
Goes without saying that you need enough transport ships to take the army (although the first method will split the army automatically if you dont have enough)