Open Steam and log in to your account
Click your profile name in the top menu
Select Profile
Look at the URL in your browser
Copy the number or custom ID shown in the profile URL
If needed, enable the Steam Console or use a Steam ID lookup website
In Steam, go to Settings
Open Interface
Enable Display Steam URL address bar if available
Visit your profile again and read the Steam ID from the URL
Use a Steam ID finder tool if you only have your profile name or custom URL