Does my employer have to pay me overtime if I work more then 40 hours a week?

Unless you are a salery worker than if you work over 40 hours in a week you get paid time and half.

Read your contract

Depends on the laws of your state, which have to be posted in your common area (like break room or time clock location) .

Some states are 80 hours a pay period.

Yes.

shall not be employed more than eight hours in any workday or more than 40 hours in any workweek unless he or she receives one and one-half times his or her regular rate of pay for all hours worked over eight hours in any workday and over 40 hours in the workweek. Eight hours of labor constitutes a day's work, and employment beyond eight hours in any workday or more than six days in any workweek is permissible provided the employee is compensated for the overtime at not less than:

One and one-half times the employee's regular rate of pay for all hours worked in excess of eight hours up to and including 12 hours in any workday, and for the first eight hours worked on the seventh consecutive day of work in a workweek; and
Double the employee's regular rate of pay for all hours worked in excess of 12 hours in any workday and for all hours worked in excess of eight on the seventh consecutive day of work in a workweek.

There are, however, a number of exemptions from the overtime law. An "exemption" means that the overtime law does not apply to a particular classification of employees. There are also a number of exceptions to the general overtime law stated above. An "exception" means that overtime is paid to a certain classification of employees on a basis that differs from that stated above.

http://www.dir.ca.gov/dlse/faq_overtime.htm

Yes under Federal law you must be paid 1.5 times your normal wage for all hours over 40 in a week. California law requires OT for over 8 hours in a day.

UNLESS you are are classified as an OT exempt employee.