[TheLastSuperman]

For that you simply need to know how to edit the home page OR edit a template and add in some basic html code and css.

Example

Add this to additional.css (template inside of the Style Manager in Admin Control Panel of vBulletin):

.mynewclass {
width:90%
margin:10px;
padding:10px;
color:#000000;
font-size:16px;
}

Now this would be what you add into the template or your new cms system:

<div class="mynewclass">
Hi, this is the home page of our forum. Lorem Epsum Dolor Emit Blah Blah Blah
</div>

That will make the box show as you wanted.

*You can remove the width:90% IF the margin i.e. the outside white-space around the box is too much, by removing the width part of the definition it will inherit the overall width of the site minus (the containers padding AND your div containers margin).