Bioware claims to have developed a "save game transfer" system, to allow you to take your Shepard from the first to the second. While porting your character, it also ports your "story". So in effect, whatever you did in the first? That is your canon.
Spoilers on a few things that will change in ME2 depending on what you did. Taken from the ME2 Wiki page
Major plot spoilers from ME1.
Spoiler Alert, click show to read:
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