The best way I know of to take part in newsgroups is to connect to them through your ISP. If you are unsure whether your ISP gives you access to newsgroups, then check around its website... you will probably find info on newsgroups there. If it turns out that your ISP does not offer newsgroup access, then you can subscribe to one of many newsgroup services found online.
You connect to newgroups through some type of software, and an address such as this: news.your-isp.com. Microsoft Outlook handles newsgroups, as do many other programs that you can get online for (usually) a small price. One specific piece of software that I have used before is called NewsBin Pro, and it works rather well. If you want more info on this program, then a simple search in Google will take you to it.