A Long-Winded Explanation
You get triple your HD at first level, plus your Constitution modifier once.
Soldiers and Jedi get 1d10 hit points per level. A 1st-level soldier with Con 12 gets 30 hp, +1 for Con, for a total of 31 hit points. A Scout with Con 12 gets 24 +1 hp, while a Noble or Scoundrel would get 18 +1 hp.
Every character, creature, vehicle, and complex object in the game has a Damage Threshold, which is calculated by taking the Fortitude defense and adding a size modifier.
The Fortitude defense is 10 plus your heroic character level plus your Con modifier, plus miscellaneous bonuses from class and armor. A droid or vehicle adds its Str modifier instead of Con, because it doesn't have a Con score, and anything larger than Medium adds a size bonus.
Darth Vader has Threshold 36, a Rancor has Threshold 36, but it's Huge, the Millenium Falcon has Threshold 76, and an Imperial Star Destroyer has Threshold 256.
What this means is that as you take damage your capabilities degrade. You have a Condition Track, which is the same for everybody, and goes from Healthy (no penalties) to Disabled (helpless/unconscious). The intervening steps are -1, -2, -5, and -10 to skill and ability checks, defenses, and attack rolls. At -10 you are also at half Speed.
Every time you take damage from a single attack that equals or exceeds your Threshold, you move -1 step along the condition track.
Luke gets lucky and tags Vader with his lightsaber. He rolls a 36 for damage, moving Vader down to the -1 position. Vader now takes -1 on all rolls involving a d20, and his Defenses all suffer by -1; this also means that his Threshold is now 35. Vader's still up, his hp are now 145 from 181, and all Luke did was make him angrier.
To kill something, you have to score enough damage to reduce it to 0 hp and equal or exceed its Threshold. If it can't spend a Force Point to make itself disabled instead, it dies.
Jabba's Rancor has 138 hp and Threshold 36. Luke hits the door mechanism and brings the giant armored door down on the hideous thing. The door does 150 points of crushing damage, which easily clears the Rancor's hp and its Threshold. It doesn't have Force points, so it dies.
A character, vehicle, etc., becomes disabled if its hp are 0, or if it hits the bottom of the condition track.
Of course, a disabled character, droid, etc. is helpless and unconscious, so you can give it the coup de grace even if you haven't ever exceeded its Threshold.