What to consider when posting in boards? How am I supposed to bevave properly?

Concerning behaviour the same rules of conduct as in the chat apply.

In addition it is not allowed to post...

promotional messages for boards not related to the chat
messages with a commercial background (sell/buy, links to eBay or the like)
medical questions as there are possibly wrong statements
personal attacks or denunciations
references or links to content not suited for users under age
questions or statements concerning other users
private talks within a thread

You should also pay attention to the following aspects:

Write your postings in a manner that other users can understand what you want to say. They cannot guess what you intended while writing, your text is their only reference. Sensible text does not automatically emerge from just stringing words together.

Try to watch your spelling and use of grammar, at least the basic rules. And also try to apply the basic rules of readability.
The ones who are supposed to read a text which abounds in misspelt words, where there are neither full stops nor commas, just capital letters ("THIS IS REALLY ANNOYING") or a meaningless mixture of lowercase an uppercase letters ("thIS iS REaLly AnNOyinG")
have a hard time understanding it.

Do also not misuse means for formatting text to "beautify" your text. This is annoying.

It is also misuse of boards to write postings which solely consist of ASCII art. These are indeed "made of text" but boards are to be used for discussions. Therefore if you want to post this kind of contribution, do so only in threads which are reserved for ASCII art.

Before opening a new thread check whether a thread covering your topic does already exist. If it does exist, do not open a new one but post your message in the thread which already exists.

Before starting a new opinon poll (provided that this function is available), please do consider whether it makes sense or not. There is no need for the umpteenth poll "Which mobile phone do you prefer?", it only is cruft.

Before posting think about a precise subject, especially when starting a new thread. The other users eventually want to know what this thread is about.

Before posting check whether others have already contributed the same textually. It causes a great deal of annoyance, if most postings only contain "yes, that is what I think" or the like.

When posting, mind the topic, so not just post anything. Postings off topic or without any text at all mostly are just an annoyance.

Ignore the board trolls. Mind the sentence "Do not feed the trolls!", it only incites them to go on. Just do not answer, it is attention they want to attract. When they are ignored it is most likely they lose interest in this kind of conduct.