How should I start My Story?

I want my story to be about a teen girl, Maci, who just moved from California to Colorado. She has a 4 month old daughter, Alleah, but no one at school knows except for the most popular guy, Reed. They fall in love, blah blah blah... Oh, and Macis ex boyfriend and baby daddy, Erik who abused her, try to come back. I know what i want to write about, but how should i start it?