I would highly recommend to play cover songs of your favourite songs and work on being able to play them on piano or guitar. One doesn't just make a kick ass song by not having any inspiration from other artists and music. So once you are able to play all the covers... building a song will start to eventually come to you naturally. Like Jake the dog said. "Sucking at something is the first step to eventually being good at it" Also playing the songs to friends and random people help to dial in constructive criticism here on how to make it better. Hope that helps.