The Gamemode Select Screen, or more simply the Main Menu, is the second screen you see once booting up the game. This is where you can pick what gamemode you want to play or the options about those gamemodes. There's also a back button, if you'd like to go back to the Title Screen.
Title screen

The current Gamemode Select Screen

Options Edit

Main page: Options

Options is a menu that allows you to change the game rules. This changes the volume, if items and Showdown Attacks are allowed or the health you start with.

Showdown Edit

Main page: Showdown

Showdown is the basic fighting mode, similarly to Smash in Super Smash Bros. You can fight between a friend or a computer player (or a harder version of the CPU). To fight with a friend, you just select player one's character, using right click, and select player two's character using left click. Once both characters are chosen, a bar will appear, saying "READY TO FIGHT!". Pressing it, (or pressing Enter) will make you go into a stage select menu. You can pick your stage, and fight. To fight with a CPU, click where it says "P2", with the blue background. If done properly, P2 will be replaced with "CPU", and the background will be gray. After that, chose your character by pressing left click, and the CPU's by pressing right click. Then the same thing will happen if you were fighting with a friend. To fight with a Hard CPU, just press it again, after P2's box says CPU. If done correctly, the box will be gray and say "CPU!" instead of "CPU". Though, where it normally says "CPU" will be red. Do everything else the same way you would every other way.

Solo Edit

 is a gamemode in Mushroom Kingdom Showdown that was added in v.0.4. Two different gamemodes are in Solo, Classic Mode and Target Smash which can only be played by one player.