Visitors to Israel need their national embassy's contact information. The U.S. Embassy is located in Tel Aviv, with consular offices in Haifa and Jerusalem. The embassy will assist American citizens, and direct foreigners to the proper consulate.
Israel is a free society, with open access to most of the country. A few areas are restricted military zones, however, that require permits for entry. A good road map, available at car rental agencies or tourist centers, will mark these areas clearly.
Since 1993, the Palestinian Authority has controlled much of the West Bank region. These PA areas are not safe for cars with Israeli license plates. Entry to them can be coordinated with the U.S. Consulate in Jerusalem.
Many buildings in Israel, including museums, shopping malls and government buildings, have security guards at the entrance. When entering, cooperate with the guards, open any bags you're carrying and permit a metal-detector swipe.
The key to Israel's security is an alert populace. Keep an eye open for unattended bags in public areas, or for people wearing unseasonably warm clothes. Alert the nearest policeman or soldier if you see anything suspicious.