For a cubic Bezier curve, I have this formula:
$$\mathrm{B}(t)=\mathrm{P}_0(1-t)^3+3\mathrm{P}_1t(1-t)^2+3\mathrm{P}_2t^2(1-t)+\mathrm{P}_3t^3,\ t\in[0,1]$$
Now about $t$ I only know that is should be between $0$ and $1$, but which value should I choose? And how does the value that I choose influence the curve?
Choosing a single value of $t$ doesn't give you a curve at all.
For every value of $t$ between $0$ and $1$ you get one point in the plane. All those points for different $t$s taken together make up the curve.
In practice, to draw the curve, you could apply the formula to (say) 1001 different $t$s: $$ t = 0,\ 0.001,\ 0.002,\ 0.003, \ldots,\ 0.998,\ 0.999,\ 1 $$ and then connect the points you get out with straight line segments. If some of the corners in the polyline bend too sharply for your taste, increase the number of $t$s until the result looks nice and smooth.