The usual way is to use a newsreader and an NNTP server. If your internet provider does not offer you access to a good NNTP server, you might consider switching providers. There is also a free (registration required) NNTP server: news.individual.net.
Once you have access to an NNTP server, you can use a wide variety of newsreaders. You should consider using one that has passed the GNKSA test, as such newsreaders can help you to avoid a number of errors when posting (even if you don't want to post right now, you probably don't want to have to change the newsreader if you ever decide to post).
Even with a good newsreader, you should make test postings to a test group (e.g., misc.test) before posting to discussion groups like comp.lang.forth. Also, check your test postings; not with your newsreader, as it tends to hide its own errors; try the "Show original" button of Google groups or use telnet.
Not only your newsreader should be well-behaved, you should be as well. Most people will do ok if they read the newsgroup they want to post to for a while before posting (note that different newsgroups can have different cultures). If you want to learn more, read the Netiquette, news.newusers.answers, about smart questions, or about avoiding common mistakes.
As a poor alternative to an NNTP server, you can also use Google Groups.