If you are coming from Golden Gate Park, keep walking until you hit the ocean. If you’re driving there is a parking lot right on Ocean Beach. If you're on public transit take MUNI's J line all the way to the end and you're about 3/4 mi south of the fire pits. Send a runner to grab bonfire supplies from Safeway (2350 Noriega St) if you don’t already have them.
Don't just start your fire anywhere. Find a fire pit and put your Boy Scout skills to work.
FYI: The water at Ocean Beach is quite dangerous. The surf is crazy and people drown all the time. Sometimes it may look calm at the top but the real danger is the undertow. I wouldn't let my kid go in past his ankles.