So you want to create a custom game or lesson in minecraft? Here are the steps.
1. First you need to create your seed. A seed is your world. Minecraft automatically generates one for you but this tutorials walks you through creating one yourself:
http://www.howtogeek.com/school/htg-guide-to-minecraft/lesson12/
2. Learning how to craft. You need to learn how to make your world. Use this guide which shows you how to create simple mining tools to buttons and levers.
http://www.minecraft-craftingguide.com/index.html
3. Once you know how to craft you need to decide what you want to do. What are your goals? There are so many things to do that its impossible to post even 1% of them here. Once you know what you want to do you need to find tutorials to help you. Fortunately the minecraft community is huge so there are tons out there. For example, here is a good tutorial on how to create a quest in minecraft:
4. Once you have created your world you need to share it. How can you do that? You can use a server or let people download your files. To share your files with others, check out the following tutorial:
http://www.howtogeek.com/school/htg-guide-to-minecraft/lesson13/