Based on a comment in this thread, it seems like you want to learn some basic music theory, to get a feel for how music is put together. Based on that, I recommended electronic keyboards as the instrument. Keyboards are laid out in a fairly intuitive manner that can help you learn how scales and chords are built, and more advanced music theory depending on what you want to learn. Also, electronic keyboards can be quite cheap, and the sound making part (synthesizers) can live in a computer and be pretty affordable as well. But don’t get a keyboard without full sized keys…that will limit your fun and have you learning bad muscle memory. You want to use a keyboard the size you would keep learning if you eventually put thousands of hours into playing that instrument.

For shopping, you want a ‘midi keyboard controller’ with a usb connection.