There are three major ways:
A ridge forms underwater where new land is created. The ridge builds over time, creating new, mountainous continents and islands, like iceland.
On the edge of a continent the ocean is being rifted upwards, on top of or against continents, creating mountain ranges half made out of oceanic sediment, as in the Alps.
A large area of land is shallow enough that, when the earth had higher sea levels, it was covered in water. The water eventually receded and then the land was scrunched up to form a new mountain range. This can be seen in the isolated mountain ranges near death valley and the great basin.
