I am a 15 year old girl. I was just wondering, if a person does something bad and that person knows it's bad but
asks for God forgiveness,  will God still forgive that person even if they knew it was bad before they did it?

God is always with us. We all do stupid, selfish, even mean things.
We are all part of God and God is part of us.  It's not important for God to forgive you.
It is important for you to learn and grow, to become a better person-- more honest, caring, and, most important, more loving.  The more selfish, bad things you do, the more you put between yourself and your connection with God. God is always there, but we can often ignore God's presence in our lives.

Forgiveness begins to come with being true to yourself, and facing your personal weaknesses and inner "demons."  It continues to grow and reach its fullness as you learn from your mistakes, your past trespasses and, in your heart, choose to act the way that is closer to love, to kindness and compassion for all of the other manifestations of God which surround you (people, places, etc... everything.)

Like I said before, we all make mistakes, do stupid things, and even, bad things. Then we learn. We fall down. We get up. We learn to walk. IF you want to take a concrete action to help "redeem" your bad behavior, then do a good deed for someone. Do it so they don't even know who did it. That way you know you did good, and you didn't do it for acknowledgement, thanks or recognition-- just for the sake of doing good.