How to make an external HTML file content not be appeared in the home paged?

How to make an external HTML file content not be appeared in the home paged?

Sorry for the confusing title, I really don't know how to title my question,
I am trying to make a dropdown shopping cart, I have designed the cart
using html css php and jquery. the file name cart.php
<div class="maincontainer">
....
....
</div>
There is a shopping cart button in the home page "index.php", to make the
cart.php be appeared as a dropdown in the index.php
I have include the car.php as include "cart.php"
First problem: all the cart.php content are being shown on the top of the
index.php even if I set the maincontainer display to none
2nd problem, I cant figure out how to call the cart in a div on index.php ?
3rd, What topic or keyword should i search, to learn how to solve the
above issues?
I am learning all my own :)
Sorry if the question is too simple.