Dexed

Install and poke around in Dexed, a free software FM (frequency modulation) synthesizer to explore more complex FM tones. It comes with many preset sounds, and you can modify them as you want. Start simple. Many interesting sound presets are available online by other people, including all the DX7 sounds. You can use the computer keyboard from A to L for white keys in 4th octave, and the keys above for the black keys like a piano. Each piano key corresponds to a musical note, and is actually specific frequency, you can google for the frequencies.