Welcome to the forum!
I don't use a tablet myself. But as far as I know, you need to set these options:
File menu > Inkscape Preferences > Input/Output > Input Devices > Use Pressure sensitive tablet (requires restart)
You might also need to check the option below that (Switch tool based on tablet device). I'm not sure exactly what that means, so I'm not sure if it's required or not.
Edit menu > Input Devices > Use Pressure sensitive tablet (requires restart)
Also, just above that, you need to set the device for either Screen or Window mode (I don't know which, sorry). If something's already set there by default, I would just leave it alone. But you definitely don't want it set for Disabled.
There may be some options you need to set with your operating system, I would guess, but I don't know what they would be.
(Geez, I might also add an faq item for this as well....)