You will need to do a bit more homework to start. Begin with what you know and fill in the blanks. When was your grandfather born? Where did he live besides Santa Monica, who did he marry? Did he have a social security card? Check out the same for known siblings. Start interviewing people in your extended family. You never know what someone may know and that someone assumes everyone else in the family knows the same things. Case in point: my brothers never knew that our grandmother had given given birth to four other children who had died in early childhood & were surprised that I even knew the names of the deceased children. I have many other examples but you need to start digging. Write it all down and when you can do the interviewing with a recorder because you can forget things or when you review the recording, you may think of new questions to ask.
In the meantime, let me know your grandfather's date of birth, your grandmother's names and her birth year. I'll see what I can find out with a basic search.