How to find the key of a song
There are several approaches to determine the key of a song, but here's the method I suggest. Have the song playing. Then take your musical instrument and start hitting single notes at a time. You'll notice that some of the notes just sound right, where as others will totally clash with the song. Write down the names of the note that sound good. Then compare your list to a list of the notes in each key. Here's a list of all major keys.
Searching through this list can be a hassle, as can knowing and writing down the names of all the notes. That reason is exactly why this site is here. Use the Song Key Finder tool to make things easier. Click the notes that belong in the song until it tells you exactly what key you just found.
As a general rule of thumb, you can try a shortcut: Most of the time the key is the same as the first note in the song. If that's not right, try the last note of the song. This doesn't work always, but works more often than not!