http://www.govmail.ca.gov/ (has a form for email, will get through faster than regular email addy)
governor@governor.ca.gov (slower but still works)
Suggestions:
Tell him you support his decision to veto the gay marriage bill passed by the legislature.
Tell him you support his courage in standing up for the rights of the voters of California.
Tell him that you are glad that he is setting a good example for other governors who may face the same issue. (Good if you are not from CA)