Should probably be in mod discussion or somewhere else, try looking there first.
Take a look at the rampage gamemode to see how they changed the player to blow up bricks.
You'll need a custom cat model, but theres lots of player-model add-ons and probably someone has made a cat already (you'd just change the player databock in the rampage mode to a cat looking thing)
Add-ons are stored in .zip files, and the most basic way to edit a .cs file (the files containing the script) would be the standard notepad program.