It's a general talk about you , why you think you want to join the Army and what you think you might like to do.
If the careers advisor thinks you are a reasonable candidate they will arrange the next steps in the selection process depending on your situation and will talk to you about going on regimental visits etc.
Prepare by getting a good general knowledge of the Army and thinking through what you would like to do.
Starting working on general current affairs etc is always a good idea - you can't start too soon.