An inexpensive 10 hole Hohner harmonica is a fun and easy way to learn a bit about music and self expression. when I was 10 my parents got me a book by Klutz Press, "Country and Blues Harmonica for the Musically Hopeless" that came with a cheap plastic Hohner harp and a cassette tape where the author Jon Gindick walks you through everything you need to know to get started on straight and cross harp. The whole tape is now on YouTube!

https://www.youtube.com/watch?v=2fayFSiceZA

The hardest skill to master on a harmonica is playing one note at a time without bleeding into the holes on either side. Once you master single hole pucker, introducing the side holes adds more texture and harmony at your discretion. The second hardest skill is "bending" by changing your embouchure and air stream flow to bend the pitch of the reed. You can't really effectively bend until you single note pucker since the reeds bend at different rates which only sounds good when you've mastered both skills which are key to a good cross harp player.

If you decide to get a harp for starters the plastic Hohner Marine Band harp is super tough and forgiving. The more expensive wooden comb blues harps sound amazing but they take some skill to master.

Diatonic harmonicas are keyed, where the "C" note (4th hole blow) can be in any whole tone, so some transpositional thinking is required once you get good. If you want to play cross harp (aka "blues") with a guitar player typically playing in the key of E, then get a harp keyed in A.