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Once you are done make a playable demo of your game - head to the Retro Plaftormer Jame home page and hit the "Submit Your Project". Note:This link will only appear once the Jam has started. 
Hitting on that link will take you to a Submission prompt


Hit the "Upload game", and create your new project. More info on uploading your First Project https://itch.io/docs/creators/getting-started

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As for Game IDEs , here's a list of free and paid services, some easier than the other 

Godot Engine https://godotengine.org/
Phaser https://phaser.io/
Unity https://unity3d.com/
Construct 3  https://editor.construct.net/
Unreal https://www.unrealengine.com
Twine http://twinery.org/
Inklewriter https://writer.inklestudios.com/
Amazon Lumberyard https://aws.amazon.com/lumberyard/
Cocos2D http://www.cocos2d-x.org/
GameMaker Studio https://www.yoyogames.com/gamemaker
GameSalad https://gamesalad.com/
PlayCanvas https://playcanvas.com/
MonoGame http://www.monogame.net/
Gdevelop https://gdevelop-app.com/
HaxeFlixel https://haxeflixel.com/
Buildbox https://www.buildbox.com/
RPG Maker http://www.rpgmakerweb.com/
Stencyl http://www.stencyl.com/
MelonJS http://www.melonjs.org/
Isogenic Game Engine http://www.isogenicengine.com/
Voxeljs http://voxeljs.com/
Pandajs http://www.pandajs.net/