First you need to purchase Minecraft from Minecraft.net. This will get you a Minecraft username and password. (This is also called a Mojang password, as Mojang was the company that originally made Minecraft.)
Instead of using the normal Minecraft launcher, use the third-party ATLauncher software from ATLauncher.com. This video (which is also on the ATLauncher web site) on the download page explains how to install and configure it.
Download ComputerCraft1.79.jar This is version 1.79 of ComputerCraft, for version 1.8.9 of Minecraft. Your browser may say something like "This type of file can harm your computer" but you can ignore this message. Browsers show this for all .jar file downloads since .jar files can be used as malware, but this particular .jar file is fine.
Once ATLauncher is installed and run, follow the instructions in this video to setup a new instance of Minecraft with ComputerCraft:
After crafting a turtle in Minecraft, place it in the world and right-click on it. The ComputerCraft GUI (graphical user interface) window will appear. It has a command line interface, the player's inventory, and the turtle's inventory:
There are some great resources on programming turtles in the Lua programming language:
The turtle needs to install a program to download programs from TurtleAppstore.com. Type this into the command line interface and press Enter:
pastebin get iXRkjNsG appstore
This will download the appstore program that you can use to download the programs you find on TurtleAppstore.com.
You can browse a list of turtle programs by clicking the Browse link at the top of the TurtleAppstore.com website. Or, you can search for programs by entering a search term in the search bar, also at the top of the website.
To download a program to your turtle from the turtle's GUI, you need to specify the username and appname for the program you want to download. For example, enter the following into the turtle:
appstore get AlSweigart mydance
WARNING: Do not use your TurtleAppstore.com password or Minecraft/Mojang password to upload apps. Passwords typed into a turtle's GUI can be viewed by the Minecraft server's admins. To prevent this, TurtleAppstore.com uses one-time passwords (OTP) to upload apps.
Programs downloaded through the appstore program have a small comment as their first line. This comment identifies the URL where the program was downloaded from. If you open the mydance program in the turtle's editor by running
edit mydance, you can see that its comment looks like this:
This marks the program as a TurtleAppstore.com program. If this program is from your TurtleAppstore.com account, you can modify it and then upload the changes. Then everyone else can download the new version of your program.
To upload a program to your TurtleAppstore.com account, you need to find your one-time password (OTP). Your OTP is on your user page. Hover your mouse over the "Hover to reveal OTP" link, and it will appear as below:
Use this password to upload any changes you've made to your program that you downloaded through the appstore program. It's okay if the admins of the Minecraft server you are on see this one-time password, since it expires after it is used once.
appstore put mydance 4P7yWC
After running this program, you'll see the following:
If you got an error message while trying to upload your program, you need to log into the TurtleAppstore.com site and check what the OTP is. Remember, the OTP is a one-time password: it changes each time you use it (this way the Minecraft server admins will not be able to modify your TurtleAppstore.com account.)