You need a digital filter with a single pole. Suppose you start with no pole and your output y(t) is just a constant, "a", times the input x(t). Then y(t) = ax(t). Now add a pole with time constant "tau".
y(t) + tauy'(t) = ax(t)
Now describe this in the frequency domain by taking the Laplace transform.
Y(s)/X(s) = a/(taus +1)
Find the discrete time difference equation using something like Mathematica's "ToDiscreteTimeModel" function or Matlab's "c2d" function. For a discrete loop time of 10mS, a tau of 1 sec, and a gain "a"=1, I get
y[n] = 0.9905y[n-1] + 0.00995017x[n]