Buy a decent one, don't waste money on a cheap one, it won't last as long.
Starting out stick with a 'C', it's what all tabs are written in.
There are normal 'C', which start the first octave on a C4, but the first octave is missing some notes, as it is mainly used for chords on a diatonic Richter tuned harp.
If you want to play melodies, a Richter tuned diatonic will work OK, but you will be playng the high notes, get a 'Low C'.
Better still is a chomatic, more expensive to start out, but has all the notes & accidentals, (sharps & flats).
My suggested makes are Hohner Special 20 & Seydel (Steel) Session for a diatonic, Hohner, Suzuki, or Seydel for a chromatic.
As I say above, it's worth spending a little bit to get a decent one, aim for an intermediate class of harmonica.
The tremolo is an aquired taste, I think, but well worth investigating, once you have the basics under your belt.
Watch out for H.A.S. - just like U.A.S. - it has a habit of creeping up on you.
P.S. If you can read music, use notation, & learn where the notes are on your harp.