Question: What do I need to know about Christian denominations in order to ask an on-topic question at Christianity.SE?
I'd say the most basic thing is the name of the denomination. Asking about what "Christians" believe is hard because there are 2.18 billion Christians in the world, and they don't have a unified belief system. However, denominations are almost always formed around a common set of beliefs.
A simple, on-topic question could be "What is the Catholic stance on X?" (provided that it's not also too-broad, etc). In this case, you are asking a specific, answerable question about a denomination that can be answered by their writings, and other published works. "Why do Catholics believe abortion is sinful?" would be another good example that shows you've done a little bit of research.
Be aware that sometimes there are classifications rather than denominational names. "Protestants" is an umbrella term for Lutherans, Calvinists, Presbyterians, Baptists, and many other groups. Sometimes you will have agreement between them, sometimes not. It can be harder to determine what the area of commonalities are.
To give some practical advice, you can start by asking questions about what the sources of agreement and disagreement are. If you have a particular denomination of Christianity you are interested in (perhaps there's a certain denomination near where you live that you're interested in learning more about), that's a great way to ask questions too. If in doubt, asking about Catholicism is always a safe bet, since they are the largest denomination and have a very unified set of beliefs with a lot of writings over two millennia.
Finally, don't be afraid to let commenters give feedback on how to broaden or narrow your scope.