To play clear melodies, you need to learn to isolate one hole at a time. There are a couple of different ways to play single notes on the harmonica, but the most useful one is the pucker technique.
1. Make big “fish lips” and put the harp in your mouth.
2. Your lips will actually be on the top and bottom coverplates.
3. Firm up your pucker and keep stiff cheeks.
4. You might need to press the harp into your mouth slightly.
Blow into the harmonica and listen. Are you getting a clear single note yet? If not, adjust VERY SLIGHTLY left or right while keeping the harp in your mouth. Press the harp in a little bit and keep your pucker and your cheeks firm.
Experiment with this and you’ll get it. Once you have a clear single note, try alternating between blow and draw on the same hole, then doing the same thing on the next hole over. Have fun!