Most safety conscience boaters will advise you to file a float plan when you are venturing out on the water. You will need a float plan when YOU want people to know where you plan to sail and you will need a float plan when PEOPLE need to locate you while you are sailing. You should leave a copy of a written float plan with your marina, and friend(s) or relative(s).
There are several online Float Plans available in various formats and within each plan there are instructions to follow in case of an overdue boater. Here are links to a few:
The BoatUS site has both Word and Pdf format
A Float Plan template from the US Coast Guard Auxiliary
The US Power Squadron has a form you can fill out and then print.
Here is the partial Float Plan that we used on our trip to Green Turtle. It changed several times and each time it changed we made sure to notify those people with whom we had shared the plan.