This might help, if not search Google for ad manager plugin
add the pop-up code in headers and ads on sidebars widgets.
Will that apply to every page?
Can i paste the same ad code for a banner on different parts of my website cuz i'll be using the same banner size for each page.
Yeah for sure.
Doing it manually in your child theme can be complicated for a beginner and if you want to change the code you got to do it again. I use a theme that has the placements already done, so it's quicker.
Adding code to header.php in the main theme's code means next update it will be lost, that's why we use child themes. Standard practice. As you're not sure what a child theme is here you go
Say i have an banner ad code. Can i put the same code on every page or do i need to get a new ad code for every single banner?
No same ad code
Yes,we've already covered that a few posts ago, you shouldn't alter code directly, always use a child theme, as covered earlier
You can use the same ad code unless you want to use different sizes. If you're using AdSense then just use the responsive code