Add a song and hit “Analyze”. It runs in your browser and takes a few seconds: you get the key (say, A minor), its Camelot code for harmonic mixing, and the tempo in BPM.
You also see how confident the result is and the runner-up. Relative keys such as C major and A minor share the same notes and can be hard to tell apart from a recording. If the song is tuned away from A = 440 Hz, that's shown as well.
Key doesn't suit your voice? Hit “Change the key” to shift it by semitones without changing the tempo.