The most basic way to take a screenshot is to use the sshot command either through a console command or via the default keybind: F9. The screenshot will be saved to your Nerf/System folder under the name Shot<FIRST AVAILABLE NUMBER HERE>. You may wish to brighten screenshots as they are taken at an engine brightness setting of 5/10.
However, there's a few other things you may wish to modify before taking a screenshot. The most common tips for screenshots follow:
Hide the HUD:
See this article. Hiding the HUD can be done by using the ChangeHUD console command or by using the default keybind F5.
Hide the Crosshair:
Hiding the crosshair can be achieved with the console command HideCrosshair. Or, you can simply choose the hidden crosshair in the Gameplay Options menu of Nerf ArenaBlast. Setting the crosshair color to Red: 0, Green: 0, Blue: 0 will also hide the crosshair. Adjust these menu values or use the same console command again to bring back the crosshair.
Hide the Equipped Gun:
This can be done by either the console command SetHand Hidden, or through setting the handedness selector to Hidden in the Gameplay Options menu in Nerf ArenaBlast. Reset this menu item or use SetHand <RIght,Left,Center> to bring your gun back.
Hide All Level Pickups:
One can use the console command Set Inventory bHidden <true,false>. However, if you are taking a screenshot to accompany your newly created level, the easiest way to set up is to hide the HUD using an above method and then use the console command Set Inventory Lifespan 1. This will remove all pickups, your gun, and your crosshair in one command. You will need to restart the level to get the pickups back.