Discretizing a continuous time signal
A continuous-time signal $x(t)$ is sampled to create an ideal discrete time signal $x[n]$ by just taking the instantaneous value of the continuous-time signal at the sampling instants. If you sample the signal at an interval $T_s$, then:

$$
x[n] = x(nT_s) = [0, x(T_s), x(2T_s),\ \ldots \ ]
$$

In your case, $T_s = \frac{1}{8000}\text{sec}$. In practice, there are no ideal samplers, so this model doesn't strictly hold, but it is close enough for most applications and is the model that you would typically see used in introductory signal processing textbooks and courses.