What Chuck33079 said. Most modern cars are very good at self-diagnosis, but they're still not perfect. Hook up a code reader, check for DTCs and pending DTCs (write them down, if present), clear DTCs, see what happens.
If car is still idling high after resetting DTCs, several DIYs have been posted on idle relearn. If you can't find one using the site's search, try any web search engine and add "site:the370z.com" to your search string. The high idle makes me think the problem may be with one of the TBs - hopefully it was just a glitch and won't crop up again.
If the P0300 reappears, then it's probably time to clean MAFs and TBs, check plugs, etc (or visit a mechanic).
The FSM is an invaluable resource for trouble-shooting the 370Z. Link in my sig.
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