If you want to add new content at main (for example, items, blocks and mobs), you should use Forge api - you can download it on files.minecraftforge.net.
Else if you want to have ability to change existing stuff's core properties, you should Minecraft Coder Pack - https://www.modcoderpack.com.
For older versions, you can obviously download Forge for 1.1. But: 1) it is not super stable 2) There are not any official Forge versions for 1.0 and older versions. Moreover, MCP is useable even for such versions like Alpha 1.2.x. You can create and edit anything you want.
You need to know that built Forge mod can just be posted somewhere and used by other players easily. MCP mods need to be pushed into jar file in the version folder and you will need to edit *version_name*.json and remove META-INF. If you will have any troubles with it, contact me here or in direct messages.
I want to make a mod for an older version of Minecraft, like 1.0. I would also like to know how to do it for the most modern version I can mod on.
New versions:
If you want to add new content at main (for example, items, blocks and mobs), you should use Forge api - you can download it on files.minecraftforge.net.
Else if you want to have ability to change existing stuff's core properties, you should Minecraft Coder Pack - https://www.modcoderpack.com.
For older versions, you can obviously download Forge for 1.1. But: 1) it is not super stable 2) There are not any official Forge versions for 1.0 and older versions. Moreover, MCP is useable even for such versions like Alpha 1.2.x. You can create and edit anything you want.
You need to know that built Forge mod can just be posted somewhere and used by other players easily. MCP mods need to be pushed into jar file in the version folder and you will need to edit *version_name*.json and remove META-INF. If you will have any troubles with it, contact me here or in direct messages.
Good luck!
I want to specifically mod Version 1.0.0, could you highlight how to mod this version?
Firstly, you need to download this (I took this link from minecraft wiki):
http://www.mediafire.com/file/s7dyeugk867no9j/mcp50.zip/file
2. Launch MC Launcher and install or launch Minecraft 1.0
3. After main menu loaded, close and copy the files from .zip file into a new folder.
4. Run decompile.bat
5. After you saw "done in %time% seconds", close command promt and open your folder as project in eclipse or intellij idea
6. Edit what you need in src/minecraft/net/minecraft
7. Run recompile.bat (testing for errors in Java)
8. Run reobfuscate.bat (getting edited classes that will be able to be read by minecraft)
When you are ready, inform me. If you have any troubles, I'll try to help.