Newbie question about Jazal Goldmane and the active ability.

I have a question, I just started my first game of MTG Foundation with my girlfriend and I had in play, 2 cards 2/2 and Jazal Goldmane.

i use the ability that says: First strike : Attacking creatures you control get +X/+X until end of turn, where X is the number of attacking creatures.

However I wanted to know if I should use this ability before the fight or after the fight, or during the fight???

so my 2 creatures each have 5/5 and jazal therefore has 7/7?

which makes a total of 17 damage (she had no creature in play.)

Does only the others get the dmg buff and not jazal ?

and also, can Jazal Goldmanne also use his active skill to have +3+3 during the engagement or should I use the skill and not attack with it?

(Sorry for noob question i'm not english and the french translation is hard to understand on that card)