I'll see what they have first, and then I'll decide.
I'll see what kind of ice cream they have on their menu, and then I'll pick one.
この場合、次のように言うと自然で良いかと思います。
ーI'll see what they have first, and then I'll decide.
「何があるか見てから決めるよ」
to decide で「決める」
ーI'll see what kind of ice cream they have on their menu, and then I'll pick one.
「アイスクリームのメニューを見てから選ぶよ」
to pick で「選ぶ」
ご参考まで!