The ladder doesn't really do anything else apart from show you your ranking, so in that way, it's pretty simple to use because it only has 1 use.
Q: What is a good rating?
A: What is and isn't a good rating generally depends on the tier and how active it is. Check the top 100 players on the ladder to get a better idea.
How to check your score
You can go the "Ladder" tab when you're at the Showdown lobby, but you won't show up unless you're in the top 100 of a tier. Another way to check your score is finish a game and your score will update chat box on the right.
You can also view all the ladders here: Showdown Ladder
Here's Smogon's guide on how to peak on the ladder: Link
Additionally, you might find that the points you gain from battles in your frequent tier become less and less, and that's because the more frequent you play in one tier, the less deviation you have and the less points you will score (presumably because you're more used to that tier so a win is more in your favour due to experience).