I am a manager for a retail store in California and recently we had some scheduling changes and I am now trying to figure out all the pertaining overtime laws.

Our workweek is from 12:00 am Monday to 11:59 pm Tuesday. Our "workday" begins each day at 12:00 am. If I have an employee scheduled to work a night shift from say 7:00 pm Monday to 3:30 am Tuesday, then schedule him to work again from 12:00 pm until 9:00 pm Tuesday what kind of overtime am I looking at?

Also, if I require an employee to work in a different store (to cover a shift for example) and that store is significantly farther (60 miles) than his home store do I have to pay any kind of "travel time"?

Our workweek is from 12:00 am Monday to 11:59 pm Tuesday. Our "workday" begins each day at 12:00 am. If I have an employee scheduled to work a night shift from say 7:00 pm Monday to 3:30 am Tuesday, then schedule him to work again from 12:00 pm until 9:00 pm Tuesday what kind of overtime am I looking at? Each 24-hour period stands alone.
So, in the example you gave :

Monday 7 pm to 12 pm (assuming no meal period) = 5 hours regular
Tuesday 12 am to 3:30am (assuming no meal period), 12 pm to 9 pm (assuming 1/2 hour meal time inclusive) = 8 hours regular, 4 hours time-and-a-half.

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

For nonexempt employees, if the employee is driving, that is customarily work time. However, you can subtract the time the employee would spend going from home to his regularly scheduled work location and vice versa. Here is an opinion letter from the DLSE regarding travel time for nonexempt employees. Although theoretically, private opinion letters are applicable only to the employer requesting them, they do relfect the opinion of the DLSE, and you would be well-advised to read this one; it seems pretty clear.
http://www.dir.ca.gov/dlse/opinions/2003-04-22.pdf