Well, there can be two options, if you have set your steamURL (a custom URL through which your friends can find you), then, you need to enter that custom URL in this page at the bottom to get your 17 digit number which is your STEAM ID 64.
On the contrary, if you have not set any Custom URL set, just go to your Steam app,
Go to View -> Settings -> Interface. Check the box saying, view steamURL when available. Then, go to your profile page and you should see your steamID-64 on the top at the address bar of the page. It is a 17-digit number.

About CS:GO:-
Counter-Strike: Global Offensive (CS:GO) expands upon the team-based first person shooter gameplay the original Counter-Strike pioneered when it launched in 1999. Two teams compete in multiple rounds of objective-based game modes with the goal of winning enough rounds to win the match.

CS:GO features new maps, characters, and weapons and delivers updated versions of the classic Counter-Strike maps like Dust, Inferno, Nuke, Train, and more. In addition, CS:GO introduces new game modes like Arms Race, Flying Scoutsman and Wingman, and features online matchmaking and Competitive Skill Groups.

Astralis - CSGO Major Circuit Qualifiers