From the main Nexus Forums page (the one you see when you access the site), scroll down to Nexus Site Forums and left mouse click on the title.
This will take you to a listing of all the game forums. Scroll down the page to the game forum in which you wish to post and LMC on the name.
This will take you to a listing of all the sub-forums for the game. Under the name of each sub-forum is a brief description of what is discussed there. Choose the one in which you wish to post and LMC on the name.
If there is no listing for what interests you, look to the upper right and you will see "Start New Topic". LMC on this and you will be taken to a page where you can start a new topic.
If there is already a topic started in which you are interested, look to the upper right and you will see "Reply to this topic". LMC on this and you will be taken to a page where you can enter your post.
WARNING: You cannot reply to any post involving a mod from the Forums. You must go to the mod page and post in the comments section. You can navigate to the main Nexus Mods page using the link at the top of any page in the Forums. The link is "Nexus Mods" this will take you to the main nexus mods page that has thumbnails for all the game that are hosted here. Just locate the game in which you are interested and LMC on the thumbnail. This will take you to the game mods page.