Action Map plugin was intended to help you identify the location of anything you wish to modify in a post/page layout.
Simon also mentioned the Hooker Plugin, a new premium product, which allows you to select the hooks from a drop down list, then paste code that's to be added to that hook. Hooker then writes the customized code correctly, for you. Once the hook is written, you can disable Action Map plugin entirely.
The question of what needs to go where is not quite as enduring as the mysteries of the Pyramids, but it's getting there! Depending on what you're hoping to add, if you have the code, Hooker provides you the chance to simply paste it and go. It's perfect, for example, for ads going into the header's branding section or contact info with linked phone numbers.
As for the display of ads on the first two posts only, being an Adsense publisher for more than a decade, I'm unclear why you'd want to do this. Adsense depends on several factors, including views and impressions. The more views; the more likely you'll get impressions. This is your conversion rate. These are usually very small, often fractions of one percent. You really don't want to limit the opportunities for visitors to see your ads.
My recommendation is to place at least two Adsense code instances (the maximum allowed is three) on each and every page or post, above the fold (meaning, before the visitors have to scroll down). One in the header's Branding area (a leaderboard works nicely here) and in the sidebar, a 336px wide ad is very effective.