The thing that confuses most people is that they see a predominantly white object and think "this must need less exposure" or a very dark object and think "this needs more exposure" but actually, if you want to render these two scenes as they actually appear the reverse is true.
The meter in your camera attempts to render any brightness level as 18% (as a simplification, look at this as a mean brightness level of a typical scene). Therefore if your subject is significantly brighter than this (like snow) you need additional exposure to render it as white, not grey - "the black cat in the coal cellar" needs less, as otherwise it will be a grey cat among grey coal.
In difficult conditions, an accurate way is to spot meter off an 18% grey card (in the same light as your subject) or use incident metering (where you meter the light falling on the subject - which is the same regardless of the reflectance of that subject).
In this digital age however it's easy to take a test shot and then adjust the exposure based on that. Generally speaking, give as much exposure as you are able without "blowing" (losing detail and texture) the highlights. This is where the "histogram" info is invaluable. Expose as far to the right as possible but without clipping the end off the graph.