Just observe the way things are done. In addition, many groups have Frequently Asked Questions lists, or FAQs, which are posted monthly. The FAQs are also available on news.answers. These lists answer the questions commonly asked on the group, questions which have been answered countless times in the past. They're designed to bring a new user up to speed without having to repeat the same answers every time a user asks.
Every group has a topic.
When posting, please make sure your posts falls within the topic of the group. If it doesn't, look for a different group that is closer to your topic. If you want to point out that you're new, simply wait until you post on a real topic and just mention it. Many sites pay for their feed and the greater the volume, the higher the cost. So, please, try not to post wastefully.