DragonBorn: Get announced DragonBorn by the grey beards, during the "Horn of Jurgen Windcaller" quest during the main questline. You can go to solthsiem earlier but most of the island will be unavailable Dawnguard: reach level 10 Hearthfire: Do a certain amount of favours for falkreath, Morthal or dawnstar
Since there seem to be a bunch of questions about this.
I quoted Pooty, since I'm lazy. People report getting DG started as early as level 1, and you can receive a letter (or not) from the Jarls of the above. Usually Falkreath.
To access it at level one, travel to Fort Dawnguard, located in Dayspring Canyon, to the far south east of Riften
Hope it helps. = )
Editted a bit. --- Remember: IamI3rian told ya. A plethora of real life problems. =' (
I've seen times that I would get a letter from the jarl of Falkreath without having visited there yet, if memory serves, I was level 4-6, definitely below level 10. Of course, I still needed to perform his miscellaneous quests.
Probably the easiest way to begin Hearthfire is, when you exit the cave from Helgen, inside that cave you can find a bottle of Black-Briar mead near the cart where the tutorial teaches you to sneak past the bear Grab the bottle, go to the jarl of Falkreath and talk to him. He'll request a bottle and you'll already have it. Next, he'll tell you to deal with some bandits. Do that and you can get started.
I also wanted to confirm Dawnguard at level 1.
Aaaand I also wanted to bump the topic, so there. --- Winter is coming