You can find your Steam64ID number by viewing your profile when you are logged into the Steam platform. You must have the URL bar enabled in Steam's settings to show this.
The URL bar looks like the screenshot below, so if you see that you simply need to view your own profile, and your Steam ID will appear as a long number at the end of the URL. Copy and paste your Steam64ID number and you're done.
No worries, it's easy to turn that on.
Click on the "view" option, on the menu in the upper left corner of Steam, and then click on "settings" in the drop down menu that appears.
On the "settings" window, click on "interface" on the left side.
On the "interface" page, find the setting for "Display Steam URL address bar when available", and click the box to the left of it.
Now you should have the URL bar enabled, and simply view your Steam Profile page, and once again, the long number at the end should be your Steam64ID.
That usually means you have a custom URL set on your profile page, and that custom URL takes the place of the Steam64ID number, so it will not show. You can copy and paste the URL shown into the online tool in the next section, and it will show you your various ID numbers associated with your account.
There are online tools that can show you information about your Steam account, and one of the more popular ones can be found at https://steamid.co/ Copy and paste the URL into the input box in the upper right corner, and it will show you all your Steam ID numbers for your account (as well as the Steam64ID number). Once you have the Steam64ID numbers that you need, you're ready to use them on your server!