Some things I learned when I came to the forums from Roblox.
1) Don't be stupid and use emotes in every post. A few is okay here and there but if you say things like, "HAI GUYS OMG LEWK ISH ISE CREEM

" and such then you're guaranteed that the community will not have a good impression of you right of the bat.
2) Take it easy with internet slang, like "lol", "XD", "rofl", and so on. If you still use them, that's okay I suppose, but don't say it when there's nothing else to say, like the example conversation.
Person1: I just read a book.
DO: Sounds fascinating. What book?
DON'T: lol
It's rather immature, and it shows you're adjusted to the Roblox community.
3) Show your intelligence. Use big words (that we can understand) to sound smarter if you have to. Most people on the site are 13 and up, and we don't take stuff from idiots who spam, like trainman1 or toxicharzard. When someone flames you, don't say "omg noob" immediately. Or at all, for that matter. At least try to make your point instead of showing you're weak, immature and unintelligent.
4) Actually read the sticky posts in forums before you post. The moderators (Rotondo, Badspot) will not hold back on you. This isn't Roblox, full of pusillanimous individual ass mods who delete everything for no apparent reason. The three big banners are posting ambiguous (unclear or unmatching) topic titles, posting something like "who cares", and posting in the wrong forum. That usually doesn't get a ban rather than a move.
5) If you have a question, ask it intelligently. Don't be like, "omg how do i do dis/" with the title being "help" because that will get you banned. Instead, ask it like this:
Title: How do I _______________?
Body: I needed help doing _____________ and I can't figure it out. Can someone help me?
6) If you're younger than most of the other forumers, act older. Don't be a goody goody and report someone because they said forget on the forums. But don't go overboard and try to hard and say it every other word.
Overall, I'd say the rule is don't be stupid.
Add your rules if you want to.