Fighting/Games

To answer your first question:
When you post in the Battlefield [normal chars] or in a board as part of the Hunger Games [Tributes only] you click 'reply' and next to all the buttons that let you do bold, aligns, colors, etc there's a dropdown list that says 'Weapon'. You select your weapon and it inserts a code which, once you've posted, will show up as one of the white boxes with the attack numbers.

I believe we rely on honor code to make sure people don't edit their posts to use a different attack, although another staff member may be able to clarify that part.
If you want to see whether someone has edited their post, just look below the main text. It says something like "Last Edit: Today at 08:32 by username." Mine is an example . Although the fact that a post may have been edited does not mean they were changing the attack; perhaps the user was just correcting coding or something. So yeah, honor code.

Second question:
It is somewhat likely to be chosen for the Games. Having more characters means more chance that one of them will be Reaped. Having characters in less-populated Districts makes it more likely that they will be Reaped. So does buying tessera.

Hope that adequately answered your questions! Welcome to the site :)