What happens if I don't import a character?
If you don't import then you will mostly follow a default path. You can choose who dies on Virmire before you start the game.There is downloadable content available that recaps some events from previous games and allows players to make story decisions.
Shepard is still fighting the Reapers. Cerberus opposes Shepard's plan to destroy the Reapers and is at war with the Alliance. Shepard is also being hunted by an assassin called Kai Leng, who is employed by the Illusive Man.
Which characters are returning?
The following characters will join Shepard's squad: Liara T'Soni, Garrus Vakarian (if he survived), Tali'Zorah (if she survived), Kaidan Alenko/Ashley Williams (whoever survived), James Vega and EDI. There is also an optional downloadable character called Javik.