You must join the server by pressing the button "Join the server" above and contact the staff, or the user that made the staff listing.
Underwatch is an Overwatch org, so knowledge of the game is required.
I'd appreciate having previous experience, but if I see that you are motivated, I might look the other way. Best would be if you did have some basic idea of how the competitive scene in Overwatch and eSports looked. Most important for me is that my staff members are autonomous, meaning they have their own ideas and can implement them. This means they have an event idea and after asking me or my fellow admins, have the capabilities of running them (of course we'll help too).