You can learn simple tunes by rote to begin with, simply watch, copy and repeat many times. At an early stage don't worry too much about learning to read sheet music, it's much more important to get playing something first although reading is an important skill which you should learn in due course. I always teach pupils to learn to play a simple tune with one hand in their first piano lesson using this method. I often use O When The Saints Go Marching In for the first tune as it can be played in just one hand position with the fingers comfortably close together. This video clearly shows how to put the right hand in the correct position and explains which fingers to play throughout the whole tune. There are pauses throughout the video for you to practise the piece in sections, you will progress quickly if you practise and master small sections rather than attempting the whole piece straight away.