[<< wikibooks] Chess Opening Theory/1. e4/1...c5/2. a3/2...Nc6
The most common and logical move in the situation is 3. b4. With this sacrificial idea white tries to make total control of the center and both the a- and b-files.
If black accepts the sacrifice a standard follow-up is:
4. axb4 Nxb4
5. d4 d5
6. c3 Nc6
7. exd5 Qxd5
8. Na3.
Other moves as Bb5 and Bc4 are standard moves in the Sicilian, where white simply develops a piece and gets a usual Silician situation.