How to make a particular blog post come first?

By default, last post comes first. Suppose if we want a particular posts to come on the main page, all we have to do is small change in the date. In the 'Create New Post Page', there is 'Post Options' link which you see, in the picture below in left side.
Click it to expand, where you can see post date and time. Now make change in the date, so that it is slightly advanced than the date of your last post. Now your desired post is shown in the main page.

Also, if you dont want to allow readers comment for a particular post, you can change it as you wish, in the left side of it.

-Sumazla.