The C# newsgroup is fairly relaxed compared with many others, but just because people won't shout at you for getting things wrong doesn't mean it's not worth putting a bit of thought into how you post. There are some issues where opinions differ considerably, and I'll try to indicate that where possible.
There are many different newsgroups under the microsoft.public.dotnet.* hierarchy, and sometimes it isn't clear which newsgroup is the most appropriate one to use. Very few people seem to get hung up about this, but for your own benefit it's worth considering where you're likely to get the best answers. For instance, if you're asking an ASP.NET question (even if your code is in C#) then the ASP.NET group is probably the best one to use.
Cross-posting (posting the same article to multiple groups) and multi-posting (posting multiple articles with the same text, each to a single group) is fairly widespread on the .NET groups, but both are frowned upon in conventional netiquette, particularly multi-posting which fragments the discussion and often leads to people wasting time answering a question on one group when it's already been answered on another. I would suggest picking a single group (after careful consideration) and posting there, then waiting for a couple of days to see if you get any replies - if you don't, pick another single group. (If you get replies which you don't consider very helpful from the first group, explain why they're not helpful on that group rather than just moving on and posting in another group - you may well get the answers you're after that way.)
There are two real choices here: whether to use the web interface or a newsreader, and which newsreader to use if you choose to use one. I would personally recommend using a dedicated newsreader, as it's likely to have a much nicer interface, making reading articles and replying simpler.
In terms of which newsreader to use, Outlook Express is one of the most commonly used on the newsgroups, but this has disadvantages:
Personally I use Gravity, which used to be commercial but is now freely available. There are many other newsreaders available, including Agent which comes in free and commercial variants.
A top-posted article is one where the new text appears at the top of the article, followed by the text it is replying to. A bottom/interspersed-posted article has each section of new text directly beneath the text it is in response to. Top-posted articles sometimes quote the old text and sometimes don't.
I personally prefer bottom/interspersed posting. It makes it far clearer which part of your reply corresponds to which part of the previous posting, and it keeps that section of the conversation in chronological order (which is very useful for people coming into the thread cold, or those reading lots of groups and thus needing a bit of a reminder about the thread.) However, many posters on the MS groups top-post and no-one gets very worked up about it. I suggest that you quote appropriately whichever way you're posting - otherwise things can get very confused.
Posting in HTML irritates some people and makes the group download slower. Some clients may end up seeing the raw HTML, and some servers which carry the group may decide to ignore HTML posts. In general, I'd advise against posting in HTML, but again the group is fairly laid back on the whole.
Attachments are rarely necessary or useful on the C# group, to be honest. Code can easily be posted inline in an article, which makes it easier to read, and anything of any significant size should usually be put on a web site and a link posted in the article. If you have captured a screenshot of an error message, typing out the message is almost always vastly preferrable to posting the screenshot (even on a web site).
First read "How To Ask Questions The Smart Way" by Eric Raymond. Please note that this is not a document specific to C# or even newsgroups - please don't ask the authors for help with your C# questions! Some of it may not be relevant here, but most is. Next, if you've got a problem with your code, post a short but complete program which demonstrates the problem - that usually speeds up a resolution significantly. Otherwise, describe what you're trying to do, and what you've already thought about or tried.
Please don't repeat your question. After you've posted, even if you can't immediately even see your question, at least wait a day before reposting it. Servers sometimes take a while to show your question, but it doesn't mean it's not there. Similarly, no-one answering it within ten minutes doesn't mean it's not going to be answered - but asking the same question repeatedly is likely to make people less likely to put the effort into answering your questions, in my opinion.
Back to the C# newsgroup FAQ.