So I need to create a discrete low pass filter. For continuous-time $x(t) \leftrightarrow X(jw)$ and discrete-time $x[n] \leftrightarrow X(e^{j\omega})$, the problem is not too difficult. But for digital signal (FFT $x[n] \leftrightarrow X[k]$), I have no idea where to start. To make things worse, the number of coefficient is limited (max 8 coef).
What kind of filter should I look for in this case?