Since Dragon Age: Origins is a series whose story/history is based on player choices, it's important to note them for the sake of not only Alistair's characterization, but for the sake of the sanity of any canonmates I may happen to get. Also note that the choices are just based on my personal preference/my playthrough of the game! Also there are SPOILERS. Obviously.

☩ THE WARDEN (player character)

✹ Female
✹ Warrior (no specializations)
✹ City Elf origin
✹ Did not take Vaughn's offer of 40 sovereigns
✹ I'm going to do everything in my power to avoid giving an actual name though DON'T DO THAT TO ME...

☩ GENERAL

✹ Everyone but Sten is recruited (because I was a dumbass and didn't know how to play this game until like 20 hours in)
✹ No changes in default specialization for any of them.
✹ Alistair meets his sister and is hardened
✹ The Warden does not romance Alistair, to keep things simple for me. Alistair did give her the rose, but she was flattered, but didn't feel the same way about Alistair. He was upset for a long time, but by his canonpoint, has accepted a very close friendship with her.
✹ idk who she's dating though DOESN'T MATTER... probably zevran

☩ REDCLIFFE

✹ The party chooses to stay and defend Redcliffe
✹ Jowan is not freed from his cell/lives.
✹ Jowan performs the ritual and Morrigan goes into the fade to free Connor at the cost of Isolde's life.

☩ THE URN OF SACRED ASHES

✹ Weylon's imposter is killed
✹ The Warden does not side with Kolgrim
✹ The ashes are not defiled
✹ Genitivi lives

☩ THE CIRCLE

✹ Party: Warden, Alistair, Wynne, Zevran
✹ Saves the mages
✹ Recruits Wynne
✹ Disagreed with Cullen.
✹ Mages are recruited

☩ ORZAMMAR

✹ Harrowmont is supported
✹ Ruck isn't killed
✹ Sides with Caridin
✹ Destroys the anvil
✹ Only the dwarves are recruited

☩ BRECILIAN FOREST

✹ The acorn is returned to the Grand Oak
✹ The offer to parley is accepted
✹ The Warden agrees to bring Zathrian for negotiations
✹ Zathrian is convinced to lift the curse
✹ The Dalish are recruited

☩ THE LANDSMEET

✹ While rescuing Anora, the Warden trusts her throughout.
✹ All of the people in the dungeon are freed/support the wardens
✹ The Warden surrenders to Ser Cauthrien. Alistair was with the group, so he and the Warden are sent to Fort Drakon
✹ Morrigan and Zevran rescue them...And not that it matters, but they make it through all the checkpoints because Zevran's story is hilarious okay.
✹ Caladrius's offer is refused
✹ The slave papers are shown to Eamon
✹ The Warden supports Queen Anora
✹ The landsmeet is mounted against Loghain
✹ The Warden wins the duel against Loghain
✹ Loghain is not recruited
✹ The Warden kills Loghain
✹ Anora becomes Queen, Alistair remains a Grey Warden

☩ THE FINAL BATTLE

✹ Morrigan's ritual is performed, alistair feels unclean forever
✹ The party that confronts the archdemon is the Warden, Alistair, Morrigan, and Zevran.
✹ Oghren leads the party guarding the gate.
✹ The Warden gives the killing blow to the Archdemon, but because of Morrigan's ritual, lives.

☩ EPILOGUE

✹ Shianni becomes Ferelden's first elven Bann
✹ A statue was erected in honor of the Wardens.
✹ The Warden decides to help rebuild the Grey Wardens' strength in Ferelden, with Alistair's help.

✹ Leliana aids the Chantry in leading an expedition to the Urn of Sacred Ashes.
✹ Morrigan does her thing. The Witch Hunt DLC takes place.
✹ Shale joins the dwarves in the Deep Roads squishing darkspawn, which she compares unfavorably to pigeons
✹ Wynne is offered the position of First Enchanter at the Circle of Magi, but she refuses it. A year later she leaves the Circle to see more of the world.
✹ Zevran remains in Denerim and help out the Wardens on occasion/when the Warden asks.

✹ Shianni becomes the next village Elder. While she often gets into trouble with the authorities for her outspoken way of doing things, she will prove to be an asset to her community. Resistance from humans saw a rise in violence in the city, however, which culminated in Shianni's murder by a human bigot several years later. The resulting riot in the Alienage forced a crack-down from the throne, a clear signal that the tension with the elves had not been resolved.
✹ Bevin becomes a famous adventurer, who tells stories about how as a young lad he met the Warden, who had used the blade to save Redcliffe.
✹ Connor is sent to the Circle of Magi. After completing his Harrowing and becoming a full circle mage, he will undertake a formal study of The Fade in Tevinter.
✹ Anora becomes a skilled governor but she never remarries due to the exceptional standard she sets for a potential groom.
✹ The Urn disappears without a trace and falls into myth once more.
✹ Following months of effort, the tower of the Circle of Magi was finally cleansed of the last spirits to slip through the Veil. No further abominations were created, and First Enchanter Irving was pleased to declare the Circle safe. All that could be saved had been.
✹ The darkspawn fall back to the Deep Roads where they plague the dwarven kingdoms once more, but leave the surface world alone for some time.
✹ Harrowmont quickly finds himself in a protracted battle against Bhelen's rebellion that leaves him unable to gain the stability he needed. The clan lords object to many of his measures in the Assembly, and only his efforts to increase the dwarves' isolation from the surface met with any success. In time, Harrowmont's health begins to fail. Some claim it was poison, while others say it is a flagging spirit. Either way, after a protracted illness, the king finally passes away. The wrangling in the Assembly for a successor begins almost immediately.
✹ a group of Dwarves try to recreate the Anvil from the remains. The first golem they create is animated by a spirit taken from the Fade. It goes berserk, killing many, and further research is banned. Interest in reopening Caridin's research never wanes, however.
✹ The Dalish become more respected due to their part in the final battle. Their connection with humans will steadily improve, but tensions will eventually rise again. If Lanaya replaces Zathrian as Keeper, she will be instrumental in maintaining the peace between the humans and the Dalish.

And as a last note, while Alistair's canonpoint is essentially during Awakening, he's not involved in the plot of Awakening, though he likely has some knowledge of what's happening.