Advice needed on improving sight reading skills

I started playing piano about 6 years ago and my parents bought me an electric piano at home, my mum never liked her piano teacher so she never made me have lessons or do grades. This means that now my playing skills far surpass my sight reading by a huge distance. For instance i can play clair de lune with little error but i can't read the simplest of songs. Any advice on how i should improve would be greatly appreciated! Thanks :)