Should i fly from Singapore to New jersey (or) Singapore to San francisco & take domestic flight?

If you just want to visit theme parks and are coming from the New York area, you should fly to Orlando in Florida rather than Southern California. The flight time is much shorter. Orlando has copies of everything in Los Angeles and San Diego (and more). There are numerous representations of the West within the parks themselves as well. If you want a little preview of what you will see in California, ride Soarin at the Land Pavilion in Epcot.

Save the West Coast for another time.

Aside from the "big three" cities in California (San Francisco, Los Angeles, San Diego), some of the must sees include Las Vegas, Hoover Dam, Grand Canyon National Park, Yosemite National Park, and Lake Tahoe.

Pick whatever combination of flights best fits your budget.

The end of the year is our winter. Plan your wardrobe accordingly, especially for the East Coast portion of your trip. (read: expect bitter cold temperatures)

If you were coming in our summer months (June through September), and had sufficient time, I could recommend a trip to the Pictured Rocks National Lakeshore, which is accessible from the town of Munising, Michigan.