Create a blog from a separate menu item
What version of Drupal are you using? If you are using 6x you will have to create a "blog" content type and fields using the CCK module. In D7 Content types and custom fields are built into core. Make sure you uncheck the "promoted to front page" option in the content type settings so that your posts won't default to the front page. If you want your blog to go to a link other then the homepage (blog for example). You will need to install the views module, and create a "blog" view page, and configure that view to display your "blog" content types as either teasers (that will further link to the full node) or to full nodes themselves if you don't prefer teasers. Then go over to menus and create a "blog" menu link that leads to the new blog view that you created. Now every blog post will post to the blog view.