If you have an analytical function and you want to differentiate it, you could use  symbolic processing:

syms x...... and other symbols

diff (f,x);

where f is your function, and you wish to differentiate it with respect to x

I suspect however from your question you just have a plot or a trace of the data ... so you could compute the derivative numerically using finite differences (and the fundamental definition of derivative is the limit of this expression as d -> 0):

f'  ~=  [f(x+d) - f(x)] / d

which simply requires the value of f at x and some small increment x+d ...

Be careful if you are trying to find max and min --- the first order test (f'=0) is necessary but not sufficient .... you have to be careful to check f" also (second derivative is negative at a max, and positive at a min)

Try to use the gradient() function.

http://in.mathworks.com/help/signal/ug/take-derivatives-of-a-signal.html