Make a post how exactly? Do you mean so that they can register and make an actual post on your blog or respond to a comment on a post on your blog?
If you want them to be able to post on your blog they have to register first to be able to do that. You would use
<?php wp_register(); ?>
which will create a link allowing visitors to register for your blog and start posting.
If you are looking to have a link to the comment section of each post you would so something like
<a href="<?php the_title(); ?>#respond">Reply to post</a>
This will direct visitors to the comment form on the single blog post. The #respond value may be different according to your blogs theme but most likely it should work just fine.
