One of the things you should ask yourself is what will you gain by telling them? Ultimately, your religion isn't anyone's business. So if you choose to tell people you should have a reason for doing so. If you just run around shouting "I'm a pagan" you're going to get negative reactions not because you're pagan but because you're attention seeking.
Personally, while I am out of the closet, it only comes up in certain circumstances. For example, when I stopped going to church I needed to explain that Christianity just wasn't my thing. I didn't even need to bring up what I *did* believe, just that I didn't belong in church.
Over time, various facts about what I DID believe in came out.
If you're wanting to tell your family, you should explain the basics of what you believe and then expect questions. Don't presume what they will ask or say. You may be surprised. Don't get defensive.
But telling them is your choice. If you don't want to tell them, and they ask, you can say that you prefer to keep your religious beliefs private, that you consider it between you and God. Because as long as you're an adult, it really isn't their business. (If you're a minor, your parents have the right to determine your religious upbringing)