We ask for IDs in several settings (shortcodes, widgets and so on).
How to see IDs in admin
Normally the ID is the post ID given by wordpress on publish.
You can see it by using the EDIT url (from admin – edit option – and looking into the url of the post/post/category you open)
ID in this example is 34
How to see IDs with a plugin
To see IDs easier you can install a special plugin. There are several to choose from.
We can recommend this one – https://wordpress.org/plugins/reveal-ids-for-wp-admin-25/
With such a plugin you can see IDs in lists and easily copy them in the widgets/shortcodes that use them: