Modify signal and define natural frequency

Hello everyone, I would like to define natural frequencies of the signals that I plotted (PSD and FFT). How is it possible to "modify" the plot in order to get more precise and clear peaks of the signals. It would be appreciated if you give me a piece of advice how to work out it. Thanks in advance!

signal1=a(:,1,1);
signal2=a(:,232,1);
Fs=32768;
subplot(211);
N=2048;
y1= fft(signal1,N)/length(signal1);
Fn = Fs/2;
Fv = linspace(0, 1, fix(length(y1)/2)+1)*Fn;
Iv = 1:length(Fv);

y2= fft(signal2,N)/length(signal2);
Fn = Fs/2;
Fv = linspace(0, 1, fix(length(y2)/2)+1)*Fn;
Iv = 1:length(Fv);
subplot(211)
plot(Fv,abs(y1(Iv))*2,'r');
hold on
plot(Fv,abs(y2(Iv))*2,'g');
title('Frequency Response-FFT')
xlabel('Frequency (Hz)') 
ylabel('Magnitude')
%red signal
N1 = length(signal1);
xdft1 = fft(signal1);
xdft1 = xdft1(1:N1/2+1);
psdx1 = (1/(2*pi*N1)) * abs(xdft1).^2;
psdx1(2:end-1) = 2*psdx1(2:end-1);
freq1 = 0:(2*pi)/N1:pi;
%green signal
N2 = length(signal2);
xdft2 = fft(signal2);
xdft2 = xdft2(1:N1/2+1);
psdx2 = (1/(2*pi*N2)) * abs(xdft2).^2;
psdx2(2:end-1) = 2*psdx2(2:end-1);
freq2 = 0:(2*pi)/N2:pi;
subplot(212)
plot(freq1/pi,10*log10(psdx1),'r')
hold on
plot(freq2/pi,10*log10(psdx2),'g')
title('Periodogram Using FFT')
xlabel('Normalized Frequency (\times\pi rad/sample)') 
ylabel('Power/Frequency (dB/rad/sample)')