I wanted to shot Tarn and had a TL and som form of torpedo. Tarn had an R7 astromech. I figured if Tarn where to use his ability to get a TL, there by enabeling him to use R7, I didn't want to waste my torpedo on a shot that would likely miss. But do I chose weapon befor or after Tarns ability triggers? Or would I be allowed to change weapon after Tarns ability triggers?

If we look at Bigs, there is kind of a precedens for picking weapon after declaring a target since you are recuqired to pick a weapon that can hit him if he is deamed in range 1 of your intended target. But since Bigs is forceing something mabye he is an exception?

You declare a target, then you decide what weapon to use.

To be a little more specific: there are 7 steps to the attack phase. Declare Target is the first. Roll Attack Dice is the second, which adds this bit of text (core rulebook, p. 11):

Instead of attacking with its primary weapon, the attacker may choose to use a secondary weapon Upgrade card that his ship has equipped.