how to calculate and plot the fast fourier transform of sla data (3 d matrix) in matlab?

I want to check for the existent dominant peaks in sla data(3 d matrix lon, lat ,time) from which I can delineate what period to choose so that I can filter this data and look for waves propagating in the region. My data is for all lat and lon(global data). However, I want to average over a lon range of 70E to 100E and after that average over 10S to 10N. This average will give a a vector only in time dimension over which I can perform fast fourier transform. Then I want to plot this with time as the x axis. I'm new to matlab, any help is appreciated. Also if anyone has done this before it would be nice if you told me if my logic is correct or not.

I have read the data as,

signal1=ncread('sla.nc', 'sla');

time1=ncread('sla.nc', 'time');

transform=fft(signal1);

plot(time, transform);

first this transform variable should not give me what I nee therefore I think I need to do the average over lat and lon..How to do this average. Also please note that the lat range is from 10 south to 10 north.