Go to your Steam profile page and check the URL, it is supposed to look like this:
https://steamcommunity.com/profiles/76561199522548077/home/
URL segment that is located between profiles and home is your steam ID. 76561199522548077 is the ID in this particular example.
Also, you can find your profile ID in the profile settings. Click Edit my Profile in the right sidebar on the profile page. In the General tab, scroll down to the Custom URL input. You will see your Steam ID in this input or in the help tip beneath it.
