Get those skill points
Skill points are very important in customizing how you want train and expand your characters. Completing missions will earn you points, but not many. Bulk up waaaaay more quickly via the achievement challenges built into the game.
Achievement challenges are available in your game menu and most of them are easily achievable (pun intended).
The best way to get a helicopter?
If you don’t want to spend cash on a helicopter then you’ll want to make your way to the NE portion of the map to find Adelaide. Complete one mission for her and you can earn the ability to call her in for transit.
She’s got one of the best choppers in the game, known as Tulip. It’s ultra fast and even has its own machine gun.