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.