Think about what you want the bet to accomplish, if you want to make him call you only with better hands then you should make it really big, if you want him to raise you, you should probably make it quite small, if you want him to call you with something worse you should probably make it an amount that whatever it is that's worse than what you have will call. This is just surface stuff, it obviously goes much much deeper, but that's not important for you and I couldn't explain a whole lot more on a poker forum anyways.