I think I can help you with this! You know, it may surprise you to learn that I've never studied any philosophy or theology...at least not in any formal manner. I've read a few religious texts, and as far as philosophy goes...the only one I've read that comes to mind is The Communist Manifesto. I decided I didn't believe in god around age 10-11 and by about age 13 I was going into AOL chat rooms to do the very same thing you're doing.
I wanted to learn about other people's beliefs, but more than that I wanted to be able to defend my beliefs when they were challenged. I wanted to be able to challenge other people's beliefs when I thought they were false...and like you, I didn't want to appear stupid while doing it. I would lie about being 13 years old and tell people I was in my late 20s just so they wouldn't dismiss my words out of hand. That's the beauty of truth though, it doesn't matter if it comes from Aristotle, Einstein, a 13yo on the internet, or Jesse2014...it's still truth and it still has the power of truth. So while you can study philosophy, theology, ethics or whatever you're interested in discussing...you don't need to. If you're willing to dive right into these discussions and arguments with an open mind, you'll eventually learn more than enough to defend your beliefs or challenge someone else's. That said, if you're still a little frightened of being insulted...there are some techniques that will help you avoid becoming the object of derision.
These are techniques that I feel will increase the respect that others have for you when posting online. When someone respects you, it's generally hard for them to insult you. Generally speaking, the basic rules of politeness and good manners which apply in real life work equally well here in online discussion forums. There are some rules though which I think should be stressed since the nature of online conversation is so different from real life.
1.
Check your tone, son. Obviously, everyone who doesn't know this has blinders on. I mean seriously, who doesn't realize the importance of tone when constructing an argument?!?
Now, if that last paragraph sounded a little condescending...that's because I was trying to be lol. It's an example of what kind of tone your words should avoid. If people think you're talking down to them, they get defensive, and they're more likely to start hurling insults your way. The tone of your words is something that can easily be confused online, since speaking techniques like "inflection" don't apply here. Tone can be used to your advantage to help convey the proper mindset you have regarding a belief...but if you're unsure what tone to use, try not to use any at all. A neutral tone is almost always a safe choice.
2.
Don't assume, ask for clarification.. If you want to reply to a post, but you're c little confused about what the poster meant, don't just jump to a conclusion. Ask them for clarification by telling them what you're unsure about. It shows them that not only are you paying attention to their words...you're concerned about understanding them as well. Most people are happy to oblige and you'll probably gain some respect in their eyes.
3.
Read the info they linked to.. This sounds pretty basic, but you'd be surprised how many people don't read links. If someone is making the effort to provide an outside resource the least you can do is read it before replying to them. Again, it shows that you have an interest in their words and they'll want to reciprocate.
4.
Admit when you're wrong.. No one likes being wrong, so no one really likes to admit it when they are. Most people will just ignore the point they were defending when it's proven wrong to them...don't be like those people. Admitting you're wrong when you're actually wrong is a powerful tool for gaining the respect of others.
There's other techniques that I'll gladly post when they pop into my head, but I think those I've provided are a good start. Let me know what you think.