How do I know what key my song is in?

[Speed Demon]

Fuzzface, the easiest way for you to establish a root key is to purchase a pitch pipe. Or a set of tuning forks.
The pitch pipe is cheaper and gives you the name of each tone you get when blowing thru it.

[Speed Demon]

Fuzzface, the easiest way for you to determine the root note of a song is to use a pitch pipe.
I recommend getting one that ranges from C to C in semi-tones.

Any electronic tuner that emits an audible tone can also be used.