I'm going to do an experiment here. Lets see if I can Build Luke from character creation and just go through the movies.
So without futher adu, Part 1. Character Creation.
So, beginning of A New Hope, I'm going to actually argue that Luke begins A New Hope as an Ace: Gunner
Here is my justifications: #1 The only personal scale weapons Luke uses in A New Hope are Rifles. At first he has a long barreled Hunting Rifle when he's going to find R2-D2, and later he uses a Stromtrooper Blaster Rifle with no problem.
#2. Luke makes the Comment about Bullseyeing Womp Rats. His position at the head of the fighter formation is making the trench run has more to do with his ability to make a precise shot.
So at character creation Luke is a Ace: Gunner.
He takes free Career skills in Gunnery, Mechanics, Piloting (planetary), Piloting (Space). With his bonus skills he takes Gunnery again and Ranged Heady.
He spends his starting XP on 3 Brawn, 3 Agility, 3 Cunning and takes 2 in his other characteristics. With his 10 remaining XP he picks up a rank in Durable, which comes in handy later when he gets assaulted by Sand People. He also spends 5 XP on Debilitating shot, and 10 XP on True Aim.
Luke spends his starting Credits on a Slugthrower Rifle for 250 (which we never get to see him use because the Sand People steal it from him before he can even fire it). He gets a Utility Belt for 25 Credits. Macrobinoculars for 75 Credits, and Heavy Clothing for 50 Credits. That leaves Luke with 100 Credits left over so that he can head down to the Toshi Station and pick up some Power Converters.