This is a very easy formatting type and is similar to doing a drop cap. The big difference is a raised initial the letter is on the base line and is higher than the other letters. Only the first letter of the first paragraph of a chapter should be raised.
Simply wrap the first letter in a span tag referencing the appropriate CSS class like so.
<p><span class="ri">L</span>orem...</p>
The CSS for a raised initial is very easy. Simply make the font size larger than normal and set it to bold.
span.ri {
font-size: 4em;
font-weight: bold;
}
The best way to illustrate this concept is with an example. Download ft-raised-initial.epub. Opening the file with Sigil you will see the example chapters and the external CSS that is referenced by each XHTML file.