Im in love with this girl named Olivia but she is dating my best named Christan what should I do

Leave her alone.

She is totally off limits while she's dating your buddy and probably for a good while longer after they break up, if they do. Date someone else.

You forget about her. She's in a relationship, which means she's off limits.

First you don't "love" her, you may like her, you may be interested in her but love is something that grows after you are in a relationship.

Leave her alone.Best wishes.