You need to get to diamond city and do the story of the century quest, then get piper as your companion

Do good things, and build your relationship. Make sure you have a high charisma. Then Romance her, eventually you'll be signifacant others.

In the game if you go to sleep with piper (as the example) you'll wake up refreshed and relaxed. Basically you had sex. If you want to have some nice time gazing at her, install the Glorious Nude mod for happy fun times