Depends if you mean playing keys or just creating sounds. Many people who play synth are usually not well trained piano players so learning piano is not a big deal with synths, but you will sound so much better if you can play piano well. As far as creating sounds, it's not that difficult. You just start turning knobs, but it's better to understand what the knobs do. Learning how subtractive synthesis works is a first good step. This is the most common type of synthesis found in synthesizers.