Search for a key phrase which would identify one of your interests. When the search returns an appropriate post, make a note of the newsgroup in which the post was found. Return to your newsreader software and subscribe to that group. After subscribing to a newsgroup, spend some time just reading the posts and learning the culture of the group. This is called lurking Once you are comfortable that you understand how the group operates, you can post to the group.
Many groups have a FAQ sheet which new users can peruse.
It is a good idea to read the FAQ sheet before you begin posting. Many of the early questions you have may be answered in the FAQ's. Have a reason to post. When replying, pay attention to the header of the message to which you are replying. Note whether the message is posted only to the group you are viewing or whether it is cross-posted to a number of different newsgroups.