If the fundamental is 100hz, then the 1st harmonic is the fundamental (100hz), the 2nd harmonic is 200hz, the 3rd harmonic is 300hz, and so on.

Sometimes the harmonics aren't exact. On a piano, if the fundamental is 100hz then the 2nd harmonic might be, say, 200.1hz or something. Some inharmonic instruments like gongs aren't anywhere close to the "ideal" harmonic series.