This should get you started:
D = load('Signal.txt');
dt = 0.02;
L = size(D,1);
t = linspace(0, 1, L)*dt;
Fs = 1/dt;
Fn = Fs/2;
FTD = fft(D - mean(D))/L;
Fv = linspace(0, 1, fix(L/2)+1)*Fn;
Iv = 1:numel(Fv);
[Amax,idx] = max(abs(FTD(Iv,:))*2);
FreqAtMaxAmplitude = Fv(idx)
figure
subplot(3,1,1)
plot(Fv, abs(FTD(Iv,1))*2)
xlim([0 5])
grid
subplot(3,1,2)
plot(Fv, abs(FTD(Iv,2))*2)
grid
xlim([0 5])
subplot(3,1,3)
plot(Fv, abs(FTD(Iv,3))*2)
grid
xlim([0 5])
The wavelength is the propagation velocity divided by the frequency in radians/time unit. The ‘FreqAtMaxAmplitude’ gives the frequencies of the peaks in Hz, so multiply these by 
to get the radian frequencies. You will then have to provide the propagation velocity in order to calculate the wavelength:
to get the radian frequencies. You will then have to provide the propagation velocity in order to calculate the wavelength: lambda = propagation_velocity ./ (2*pi*FreqAtMaxAmplitude)
