I want to center the text for my Blog Title and Tagline in the header.
How can this be achieved?
I have unsuccessfully searched for a means of setting a left margin for these texts.
My blog is the-quantumfactor.com

How about using text-align:center for .blogtitle and .blogtagline?

I have tried text-align: center
I have my width set to 990px
text-align: left sets the left side of text approximately one third of the the way across and text-align: center uses that same left margin.
How can I reduce that left margin?

Take off the float left and set the right and left margins to auto.

Where do I do that?
Sorry - I am still a bit of a beginner.

Go to "Atahualpa Theme Options", then scroll down to "Add Html/Css inserts" in the 1st box paste the following

<style type="text/css">
div.titleoverlay {
    float: none;
    margin-left: auto;
    margin-right: auto;
}
</style>

I suspect the code that is causing the issue is in the theme options somewhere. Editing that would be my first choice to fix. You could also use the code kangcarew gave you with a slight modification and just put it in the CSS Inserts box in that same section. You don't need to include the style wrapper in that box. It could just be

div.titleoverlay {
    float: none !important;
    margin: 0 auto !important;
}

Thanks Larry and Kangcarew.
Both methods work great.
Where can I get the instructions to sort this out myself?
Is there a guide for this type of coding?

Cheers
Reg

Take a look at the css tutorials at w3schools.com/css

I've found that installing firebug for firefox and looking at the specific elements on your page while messing with the code allows you to check by trial and error without endangering your code. It's also helpfull if you see something cool on another site and you wonder how they achieved it.