I've just put together a computer for the first time. I've found plenty of info online that covered the construction, but now I'm ready - I think - to power it up I'm wondering if there are any checks that I can do to be sure that the machine is safe and ready to turn on. What are the things (if any) that I absolutely must make sure of before turning on?

Just to be clear, I'm sure there are many things I could get wrong that would mean the computer won't work properly, but only a few that might cause serious damage. It's really the latter I'm asking for.