Often you will find certain themes and plugins settings give you options that let you hide or display certain posts, pages, categories and tags. But they typically ask for the ID. Often, when you install WordPress you start with the default permanlink, which includes an ID number. Once you change you permalink, you no longer see it.
Best and safest way to go is:
ADMIN PANEL -> POSTS -> Click on post you are looking for (same applies to pages) and you will enter page editor window.
Once you are there, pay attention to browser URL, and you will see something like this:
In this case, your post ID is 1322, which comes from
post=1322. In page editor same URL variable hold value of Page ID.
If you are looking for quicker way to find IDs go:
ADMIN PANEL -> POSTS
From list of all your posts, hover over EDIT button of desired post you are looking for:
At the bottom of your browser you will see URL containing desired info:
If you ever need category ID or tag ID use same logic. One is for sure, everything you need is in admin panel.