Context: Shuri Kojuurou 朱里小十郎, a boy who looks like a girl, fell in love with Fujinomiya Neko 藤ノ宮寧子, and now he's trying to look more manly in order to impress her, starting by changing his first person pronoun from boku to ore.
koko de kimeru-n-da! ここで決めるんだ [I have to get it right] here! (it's now or never!)
kimeru 決める To settle. To decide. To score a goal. To carry something out successfully.
"boku" janakute "ore" 「僕」じゃなくて「オレ」 Not "boku," "ore."
