What is the first step in collecting on a debt that is owed to you?

Well you missed the first step you should have taken, which was to file a mechanics lien.  You can extend the time by extending credit, but once they breach the credit agreement the time starts ticking again.  I'd have to look and see if there have been any changes, but I'm pretty sure your deadline was 90 days.  You should contact an experienced construction attorney directly immediately.

First, you can try simply calling and/or writing (preferably some way you can prove delivery). Remind the client that he owes you the money, and warn him that you will take legal action if necessary. Personally, I call first, then follow up the calls with writing.

If the client won't pay, your only option then would be to initiate a law suit to collect. If the amount is under the threshold for your local small claims courts (contact the court to find out, as well as to find out timelines, procedures, forms, etc.), you might consider bringing the action yourself in small claims court. If it's over that threshold, you'll almost certainly want an attorney.

Note there is no mechanism to collect from a recalitrant client (or indeed, anyone who owes you money) other than to sue.

As an option, you could try to contact some collections agencies to do the work for you. Some will do it for a percentage or cut  of what they collect; some may charge some fee instead of a percentage or in addition; there are even some that buy the claim from you for dimes on the dollar. Going to a collections agency is a good option for businesses that have alot of collections; however, since you end up paying the agency in one way or another, if you can manage the collections effort yourself, you'll probably make back more money.

You'll have to file a lawsuit against the client, unless you can negotiate something else with him,  suing on the original contract as modified by the later agreement.  If you think that the client is likely to dispute the amount, or that the work was completely properly under the contract, I'd strongly recommend a lawyer's help.

Going forward, if this is how you earn your living, you should talk to an attorney about the mechanic's lien law, and what you need to do to use it, because at least in some situations, it can give you a much stronger means of collecting.  It's too late to do that with this one.