To start developing Lua mods with Eclipse LDT you first of all need the program. You can download it from here. You could start developing instantly after installing LDT but setting up Diorama execution environment might be useful. The execution environment provides documentation and auto-completion for Diorama functions.
Debugging is work in progress.
Start off by downloading the execution environment from here. Extract the zip file. Start up LDT and create a new Lua project. Click "Configure Execution Environments" and "Add". Locate lua-5.1-diorama.zip and select that. Set target grammar to lua-5.1 and you're good to go.