Does anyone know how to get a google ad right after my header, and before any of my posts.

website located here : S&P 500 Trend Analysis

Thanks!!!

Use CSS

#googlead{
float:right;
}

Now, call the div where the google ad is, "googlead".

You could learn some knowledge from wordpress offical site. Of course you should know HTML too.

Just a tip, people clink more if the ads are located somewhere in the content of the blog as well as sites and so on. But if you still want your ads after the header, as others said: look up in your HTML Code and find where ends the header and begin the body of your blog. I see that you done it, anyway good luck with the promotion of your blog.