Sam Alexander is a relatively new character to the Marvel universe but he has made several appearance in comics as well as his origin being told in his own series.
Sam appeared first in the Marvel point one where he made a short story appearance but this was more seen as a preview to Avengers vs X-men. Sam then appeared in an Infinite comic that coincided with the release of Avengers vs X-men and lend into the events of the first issue. Sam then played a pivotal role in the climax of Avengers vs X-men where he took down Cyclops briefly.
Sam made an appearance in the Marvel Now! point one where he got into a fight with Diamondhead and this was a little teaser for his new series.
The best point to start reading Sam in his new series (Nova) which is currently on a 5 issue story arc showing his origin and how he became Nova.