If you select a ground vehicle in your hangar you will in the lower left corner of the clipboard see "Fly" and then a black square. In the center of this square is a button labeled "H". Pushing this button will start your vehicle at the base.
At some bases there are other buttons, some of the marked "S" (south), "E" (east), etc, or just blanks (instead of SE for instance). Pressing these will make you spawn in that direction, always towards another base close to the one you are at. Look at the map and compare the spawn options. It's mostly obvious what field each button corresponds to.
PS: If you are a plane the buttons are which direction you wish to be facing at the field you are at when you start.