To play clear melodies, you need to learn to isolate one hole at a time. There are several useful embouchures or mouth positions, but I believe the simplest and most flexible one for beginners is a pucker.
1. Push your lips out and make a vertical oval.
2. Place the harmonica between your lips and press in slightly.
3. Keep your pucker firm!
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. Remember to 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!