The word “resolution” is used in two different ways in DSP. And this causes confusion. Some people use the word resolution to refer to the FFT’s bin spacing (measured in Hz). I don’t like that use of the word resolution. More correctly, other people use the word resolution to refer to the ability to “resolve” (identify) two different independent spectral components or signals. Now Max is correct. The ability to “resolve” (identify) two different independent spectral components is always proportional to the length of your time-domain signal. If you want to resolve two closely-spaced (in frequency) spectral components you must increase the length of your time-domain signal.
Keep in mind, zero-padding will reduce your FFT bin spacing but will NOT improve your ability to resolve two different independent spectral components.