How to be best friends with somone?

Well, there's Lucy at my school & she fell out with her best friend Amy but then they became friends again but says things about Amy but then at school talks to her and acts nice to her. But Amy is very bitchy and selfish to everyone except Lucy. Now I'm already good friends with Lucy but I want to be best friends with her & I feel Amy is getting in the way of that so how do I get Lucy to be my best friend?