You need special privileges to add a forum topic. Go to create content > forum topic to obtain the form shown below. This form is also identical to that of a content page, and is completed in much the same way. A forum topic is the start of a threaded discussion

• Enter a subject. If this is the first forum topic to be attached to a forum, then give it the same name as the forum to which it is to be attached. Otherwise give it a concise name
• From the forum drop down box, select the forum, the topic is to be attached to.
• If the menu setting option is not expanded, expand it and provide the topic with a menu link title. Attach the forum topic to the menu structure.

This site requires all nodes, and hence forum topics, to be attached to a menu. You site requirements/policy may differ from this, relying perhaps solely on the taxonomy for navigating forums.

• Complete the body section as you would an ordinary content page. Because this is a forum topic you should aim to keep it short. Provide a brief outline of what is to be discussed. usually, but not necessarily, this is presented as a question.
  Place the cursor at the end of this first paragraph, and click the Split summary at cursor to set the teaser length. This is especially important for forum topics, but its also a good idea to get into the habit of doing this for ordinary content pages as well; so that any constructed views do not cut off in mid sentence for example.
  For similar reasons, do not ass images into this first paragraph, keep them to subsequent paragraphs where you can add further information as required.

Complete the remainder of the form, as you might for a content page, but pay particular attention to the comment settings. When finished save the form

Comment settings

There is one final option to consider, and that is the comment settings option.

• In most cases you want members to be able to read and add comments, and this should be the default. Check that the comment option is set to read/write.
• If this forum topic is the first one being added to a forum, which is to be split into a number of sub forums, you may decide to direct all comments to one of the sub topics in which case, choose the disabled comment setting.
• Lastly after some time has elapsed you decide to close the discussion down. Revisit this setting (in edit mode) and change the setting to read only; to prevent any further comments being added.
  Usually the last comment, or reply, should indicate the discussion is closed

If you haven't already done so, create forum page and locate the forums page in the menu structure of your site. Usually this page will be a (top level) primary link, as it is a prominent feature of any site.

Alternatively once this page is created, goto administer > site building > blocks, and locate the forum block. Place this in region you want forums to appear