I will probably be filing a small claims in CA and need to know where to start. It is for a security deposit not being returned after the 21 day period. It's $4200 deposit.

You walk into their office, tell them you want to file a claim, they give you a fill in the blank form, you fill it out, give it to them with your money to file.

I would start by calling a clerk in the small claims office. I don't know what the amount in CA is for small claims. You might not be able to file in small claims and have to go to superior court. If it is small claims, take a copy of your lease to the clerk's office together with any other proof you have, and they will help you fill out the paperwork. You will have to pay a filing fee.

Most small claims courts have a web site that details what you need to do. So start here:

California Courts: Self-Help Center: Small Claims: Small Claims Basics (http://www.courtinfo.ca.gov/selfhelp/smallclaims/scbasics.htm)