Posts are ordered according to latest posted first on page. Hence the newest posts are first with earlier posts lower down in the order.

Your blog consist of a Main Page and Post Pages. The Main Page contains the number of posts you have set in Settings > Formatting > Show Posts on Main Page. The rest of the posts are shown as links in the Blog Archive widget in the sidebar. The posts on the Main Page also have their links in the Blog Archive widget. Each Post Page contains only one post.

To change this order login to Dashboard. Click on Posts link after 'Manage' under the name of your blog. You will see a list of all your posts with links to Edit, View or Delete them. Click on Edit link of the post you want to change the date.

The Posts Editor opens with its two modes: Edit HTML and Compose. The second mode is present only if you have enabled it in Settings as described in Set Settings. The bottom frame of the Posts Editor contains a link 'Post Options'. Click on it to expand the frame as shown in the image. In the expanded frame you can change the Posts Date and time.

If you want this post to be the first post always in your blog then set the date to any time as far in the future as it will allow it to be set. This is because once that day arrives and you post more posts this post will start migrating downwards like the rest. Till then it will remain a "sticky" post at the top of your blog.

This is frequently used by bloggers who wish to post a book with chapters and all in their blog and want the first post to be a "Table of Contents". To see an example see here. Also you should have Post Pages enabled by going to Settings > Archiving.

If you have made a post like a 'Privacy Policy' post which you do not want to show on the Main Page backdate it. Then it will appear as a link in the Blog Archive widget.

(If you have any question, click the HELP tab)