Get the look right. Hopefully you have some kind of look – colours, fonts – in mind and ideally a header image, even a logo.

Select a likely theme as a basis.  See Using Themes and Theme Development for reference.

Work on the style.css and header.php and footer.php files to get the fonts, colours, layout and header (and footer) appearance as you would like them.  These can all be found in /wp-content/themes/mytheme/ within your WordPress subdirectory (where mytheme is what you’ve called your version of your selected theme – remember to keep an unchanged copy for backup).

Designing Headers is also a helpful reference.