My mercury has no fire HELP

[<GeraldB>]

My mercury mid 70's has no fire. Where do I start please help???Mercury 500 50hp

[<SkeeterFin>]

First is to pull the plugs and check for spark. I assume you did that part.
Next would be to make sure the dead-man switch isn't tripped (if you have one).
Third thing to do is disconnect the black wire with the yellow stripe back by the ignition module. If you have spark now, then it's a harness/switch problem. If not, you'll need an ohmmeter and a manual to test components.