copied from the game guide, some highlights, sorry i dont' have more info but this is a start. I did NOT write this, I just typed it up from the strategy guide for a friend. Take it for what it's worth.
Spoilers (mostly for the romance, I didnt do much quest spoilers because I wanted my first run to be pretty much blind)
Romancing Fenris
Firstly either favor Meredith’s views in the Mage vs Templar quest lines and side with the templars. Alternately, leave him at home when you do those particular quests.
If you want to secure a rivalry relationship, use artful answers when you DO the templar side of the quests.
Crisis Points: you will lose Fenris if you do (or fail to do) the following
* Fail to do the Bait and Switch Quest (which introduces him)
* Refuse to go to the slaver camp after the second Tevinter ambush in A Bitter Pill
Substitute Fenris from the party after the first ambush which triggers A Bitter Pill, then fail to resume the quest. After a certain time the outraged elf will confront Hawke and leave the party.
* Allow Danarius to take him during the Alone companion quest.
You must complete his companion quests, reach 100 Friendship or Rivalry and complete his final stage relationship conversation, Fenris will abandon the party after the destruction of the Chantry if you support Orsino. He can be persuaded to rejoin during negotiations at the Gallows if you have a solid friendship with him. If he does not return to the fold, those siding with the mages must fight him in the templar hall.
Romance
Reach 50% in friendship/rivalry and flirt with him during the Questioning Beliefs midstage relationship conversation. If it stays at least 50%, Fenris will visit the Hawke estate at night. At that point, flirt and it triggers the romance and you can ask him to move in.
Complete remaining companion quests and dialogues, give him his gifts, reach 100 in friendship/rivalry and complete the final stage conversation in Questioning Beliefs BEFORE the start of The Last Straw.
If you are on the rivalry path, you must side with the templars. If you are on the friendship path, you speak with him at the Gallows Corurtyard or Gallows Prison during the Last Straw to complete the romance.