which

I decide.

私は決めます

I decide today.

私は今日、決めます

I decide with my friend today.

私は今日、友達と、決めます

I decide with my friend today after comparing prices.

私は今日、友達と価格を比べた後、決めます

I decide with my friend today which shop to buy from, after comparing prices.

私は今日、友達と価格を比べた後、どの店で買うかを決めます