manage adsense on my blog

hi, my blog is https://electricguitartips.net. Right now i have adsense with size 336×280 which appears at the end of every post. Plus 1 on the header and 1 at the sidebar.

Actually what i want to do is to manage my 336×280 adsense appears only at the end of the top post. not at the end of every post. But I dont know where I should put the html script code.

Can anyone guide me?