To make a class fighting game like Critical Strike in Roblox, you will need to learn how to use the scripting language Lua in RobloxStudio. You can start by creating a GUI with buttons that will allow players to select their class, and then write a script that will execute certain actions based on which class the player has selected. You can also use the built-in animation editor in RobloxStudio to create class-specific animations. There are many tutorials and resources available online to help you learn how to script in Lua and create games in Roblox.