can i petition for my half brother? My father's son.

Firstly only US Citizens can petition for parents and siblings.

NOTE: Siblings have the lowest priority for visas some wait up to 20 years for a visa number, it would be quicker if you first become a citizen then petition for your father, and then when your father comes over he can petition for his son.

http://travel.state.gov/visa/immigrants/ty...types_1306.html

The directions for I-130 makes no distinction between step half or full siblings.