To create a counter, start with a Thread Group, right click it and select Add > Config Element > Counter.
This will present the counter configuration pane. The Name is just what will appear on the left in the Test Plan, the important value to set is the Reference Name, this is the variable name that the counter will be referred to in the rest of the project.
Other variables should be configured as desired with a start, increment and maximum parameters. If Track counter independently for each user is ticked, the counter will be a per-thread counter instead of a global counter.
To use the counter, for example in a SOAP/XMP-RPC Request simply refer to it by its name, like so: