Attraction - When waking after Eden Prime, Shepard will find either Ashley Williams or Kaidan Alenko waiting at the Commander's bedside in the medical room, feeling guilty over getting snagged by the Prothean Beacon. Shepard can be reassuring and claim there was no way to know what the beacon would do, at which point Williams or Alenko will look much happier and a little intrigued. Courting - After completing the first primary mission on each key world - either Therum, Noveria or Feros - Shepard can speak to either Ashley or Kaidan and begin building up a rapport that gradually grows into a romantic interest.
Sacrifice - On Virmire, Captain Kirrahe will ask for one of the squad members to join his diversion effort while Shepard infiltrates the base. Both Ashley Williams and Kaidan Alenko will volunteer, and Shepard is forced to choose one of the two potential love interests to risk in the assault. Later, near the end of the mission when Saren attacks, Shepard will be forced to choose which squad member dies in the nuclear blast. Sacrificing Shepard's current love interest will end that romance path. Lockdown - While there's really no time to woo the commander's romantic interest during the pursuit of Saren, they will come to Shepard's side after the argument in the Council Chambers. Only a decisively Renegade conversation path will negatively impact the romance at this point; otherwise the romantic interest will comfort and reassure Shepard. However, the intimate mood is immediately shattered by Joker with an urgent call over the ship's comm. Conclusion - If the romance subplot has been continued, while en route to Ilos the love interest pays a private visit to Shepard's personal quarters aboard the Normandy. Paragon choices will make for a passionate ending and an intimate cut-scene. Completing the romance subplot unlocks the Paramour special achievement.
That is all i know.I took it from Mass effect wiki.