Generally, all you have to do is open your driver's side door and look at the little sticker. Ya might need a magnifying glass for this. It will tell you The tire size that is supposed to be on your car. It also tells you the tire pressure that you are supposed to have in said tires.
Now it gets tricky........If it's only you in the car....Go with the recommended tire pressure.
If the significant other is with you, 2 more pounds in the passenger side. If she is fat, 4 more. (Gotta keep it level)
Now, In case of snow, what you really need is a few of her friends.....If you put enough weight in the poor car, it will go. It'll be wheezing and farting, but it will go with enough weight in it.
No reason to adjust the tire pressure, just adjust the unsprung weight....till it won't move.
In all honesty though, The tire manufacturers in this day and age have a good handle on what their product will do. Just keep them at the correct pressure and you should be good to go.