petrophysical crossplots
Short answer: it depends.

The usual workflow is, very roughly, something like this:


Condition all your data. Compute any 'missing' data you need or want (e.g. porosity $\phi$, lithofacies, saturation $S_\mathrm{W}$, S-wave velocity $V_\mathrm{S}$).
Inspect crossplots of relevant properties like velocity vs porosity, acoustic impedance vs elastic impedance (or maybe Poisson's ratio or ${V_\mathrm{P}}/{V_\mathrm{S}}$), and so on, colouring points by depth, $S_\mathrm{W}$ or lithofacies, say. Look for helpful discrimination. 
Use judgment, research, advice, or inspiration to decide on how to use the relationships you find... perhaps you are aiming for seismic inversion, or facies prediction, or log modeling, or something else.


Before doing much else, I recommend picking up Avseth et al. (Cambridge, 2005) (and/or perhaps  Dvorkin et al., Cambridge, 2014) and reading it from cover to cover. Here are some short reviews and more details on these books. Alessandro Amato's two recent tutorials in SEG's Leading Edge will help too, especially if you're using Python. If you're in MATLAB, a lot of the code for those books, which are both by the rock physics team at Stanford, is available online.

Here are some nice $V_\mathrm{P}/V_\mathrm{S}$ vs acoustic impedance $I_\mathrm{P}$ crossplots from Amato's second tutorial to inspire you: