I recommend you start with a harmonica in the key of C for several reasons. Most importantly, a C harmonica will enable you to be in tune with the majority of beginning level harmonica lessons.

With regular deliberate practice, you can expect to be playing simple pop tunes within around 3 months. Within 6 to 12 months, your technique will improve and you will probably be able to work on bending notes (a very important skill for getting the best out of a harmonica).

So, you're a beginner and want to know which type of harmonica to buy? The short answer: buy a 10-hole diatonic harmonica in the key of C. Why? Because, most beginner lessons, and all of the lessons that I teach on YouTube and in my Beginner to Boss course are on the 10-hole diatonic harmonica, in the key of C.