You have two options:
a)
You do it like this:
<div style="text-align:center;">Your content that is to be centered goes here</div>
In this case you need to do this once to your navigational links in the header and once around the bookmark icons. If you use a plug-in for the bookmarks, it depends on the plugin you use.
b)
Or you do it via your css. In that case, you begin like before, only this time it is:
<div class="pick_any_name_for_your_class">again, your content goes here</div>
and add this to your stylesheet (anywhere in your stylesheet, preferably at the bottom):
.name_of_your_class { text-align:center; }
The latter has the advantage that you can also use this for future changes, simply wrapping the <div class="name_of_your_class"></div>
around anything that can be centered (works for me on text, pics, embeds, javascript, iframes), wether it is part of the design or a post (unless you write posts using the visual editor).