How to Show A OFDM Symbol's PSD in Matlab
If you are looking at the sidelobes in particular then you will probably want to oversample, as Deve mentioned.  x2 oversampling should be sufficient, but you can do more if you want.  I generally plot my FFT's as follows to get the energy in a dB scale (Matlab syntax)-

plot(20*log10(abs(fft(dataVector))))


or, to center the FFT visually-

plot(fftshift(20*log10(abs(fft(dataVector)))))