Account Info
How to get account info for a Steam account.
API Key
You can obtain an API key here.
Steam ID
A Steam ID is a unique identifier for a user on the Steam platform. It is used to identify a user when making requests to the Steam API.
An easy way to find your steam ID is to visit Steam ID Check and enter the link to your Steam profile.
Alternatively:
Open your Steam Client and in the topmost menu go under "Friends" -> "Add a friend"
When it opens it will show your User ID as "Your Friend Code"
Your friend code is your Steam ID.