Open Steam
Click your profile name in the top-right corner
Select View my profile
Check the URL in your browser
Your Steam ID is the long number in the URL if it is shown
If the URL shows a custom name instead, use a Steam ID lookup tool
In Steam, go to Settings
Open Interface
Enable Display Steam URL address bar when available
Reopen your profile and copy the Steam ID from the URL
In-game, open the Steam overlay with Shift + Tab
View your profile there to find your Steam ID
Ask a friend to open your profile and copy your Steam ID from the profile link