Each birthsign has a menhir stone in the world. Interacting with one changes your birthsign to the stone's sign. You can use creative command: '/getbirthsigns positions' to reveal all locations
The Warrior birthsign
The Ender birthsign
One of the options is to have a randomly chosen birthsign on first login, or to pick from the birthsigns UI (B button by default)
The Mage birthsign (Electroblob's Wizardry magic)
The Blaze birthsign