this is very simple to add the adsense html code into your wordpress website header.
just follow the simple step to paste adsense code right after the Head tag
Step 1:- login your wordpress website
Step 2:- Go to left appearance menu -> Editor submenu
Step 3:- find the header.php page from right side and open it.
Step 4:- when the header.php will open you can see the tag <head>
just paste Your adsense code and paste below the <head>tag
the click on the update file button