beat some missions and then u will unlock 1 by 1
you can do things
just keep donig the missions and beating them.. pretty soon you'll get to go to other islands.
unlock it
012435
in the desert, you have to unlock it
complete the missions
pass the first mission
You cant
when you get to it
no.
You unlock it later on in the game as a mission.
Easy; play the story. If you're having trouble, (I hope not) then you can use cheats, but be forewarned that some achievements will be blocked.