This page contains all the information you need on how to have a romance with Samara.
Prevent Samara from Dying
Samara is a romance option for both Male and Female Shepard in Mass Effect 3 if she survived the Suicide Mission in Mass Effect 2. However, the option to romance her heavily depends on if she survives the mission Kallini: Ardat-Yakshi Monastery. To ensure she survives, make sure to take the Paragon interrupt when she points the gun towards her head.
If she survives, meet with Samara at the Citadels Presidium Embassies and speak with her to progress the relationship.