hello i have a website http://bestbabyshowergifts.org/ i would like to add 728x90 on top of header how can i do that please i try to add the adsense code inside header.php in wordpress but it comes above the space that i want !
The easiest way to do that is to use a plugin. I use WP-Insert: http://wordpress.org/extend/plugins/wp-insert/ It works great and lets you insert banners almost everywhere on your template, without actually doing anything on your template.
While sometimes plugins are the best choice I feel like that the area you are talking about needs to actually be manually put in. Some themes allow you to have a header ad while others do not have this option. Just do a quick search on Google to see how to add the ad to the header, if not you might have to pay someone here to do it for you. Each theme is different so that I cannot give you the generic instructions and expect you to do it.
Use WordPress plugins like QuickAdsense, Header and Footer, Wp Bannerize etc. Sometimes some plugins didn't work well with your theme. Try different plugins.