You can find your 17 digit Steam ID by doing the following:
AUTOMATIC METHOD (EASIEST)
Login to the widely used website steamid.io (click here), click your Profile picture -> Profile and copy your 17 Digit code (steamID64).
MANUAL METHOD (30-SECONDS)
- Open up your Steam client and choose View, then click Settings
- Choose Interface and check the box that reads, “Display Steam URL address when available”
- Click OK
- Now click on your Steam Profile Name and select View Profile
Your SteamID will be listed in the URL at the top left (it’s the really long number at the end.)
Steam ID: 76561198240190864
If you have a custom URL, the number will not be listed!
In that case you’ll need to paste your custom URL into this website to determine your 17 digit Steam ID: https://steamidfinder.com/