How can I file a lawsuit to collect on a loan that is due if the debtor lives in Canada

You could first try having an attorney sending a threatening letter. This occasionally works, especially, if you are willing to accept less than the full amount. If this does not work or you do not want to go that route, then you could file a lawsuit in small claims court in your county (assuming the money was loaned while you lived in California). You would have to arrange to have him served in Canada. There are very specific processes that must be adhered to when serving someone outside of the US. You should find a local attorney who is familiar with the requirements.