Do I need a chain guide or bash guard to convert to a single chain ring?
I agree with Batman.  You can disconnect your front shifter, and use the indexing screws to position the front derailleur to act as a chain guide, and you're good to go.

If you're not interested in doing that, I would recommend paying attention to Emyr's advice--chainrings intended to be used with a shifter have "ramps" on the gear teeth to facilitate shifting.  If you're not using a derailleur, these can make your chain drop, which is bad.  You might want to get a singlespeed-specific front chainring, they're not expensive.  

But, if you keep your derailleur as a chain guide, you'll be fine.