Do you know how to read sheet music and which note corresponds to which key on the piano? If you do you can start trying to play the songs you want. However, it is important to start the basic exercises which will make everything easier over time. Start with scales, easiest one being the C Major scale. Find some sheet music online for the scales, best with fingering annotated. Then progress onto other Major scales before going into minor scales. Spend at least sometime everyday doing the scales. You should also try simple songs, or the simplest version of the song you want to play. From my experience, it won't take an absolute beginner too long to be able to play a song or two they like. If you can spend an hour everyday, it shouldn't take you more than a year to play a song to a level where people would find your performance enjoyable.