Depending on what you are attacking it to, you need a 2-3 inch long screw and possibly anchors.
The bracket has two screw to be used in the two holes on the flat part of the bracket which is against the post on pointing toward the ground.
Absolutely. I believe it also came with 2 screws.
Yes you can if mounted properly. Use anchors with the screws depending on type of material you are attaching to. At least 2 inch screws.