If i want to do modding.

If by "modding" you mean "scripting in LUA", then you should start learning LUA

Unfortunately, I haven't really any experience in LUA myself, but http://www.lua.org/ would be a good place to start

also @Babas stack overflow is a priceless resource for scripting such as lua and here's a link to their site: http://stackoverflow.com/questions/tagged/lua

I think for the right terminology in this case we can say serverside = scripting and clientside = modding.
So which of them you use depends strongly on what you want to do. In the assumption that you say "Hello" to players which join you server, or to allow them to use commands like "/give food" or "/claim here", you should be able to do it all in lua. The modding would be more to add complete gameplaymechanics including the user interface and its logic. But this boundaries blurs out, you could do something like "steamworkshop" where people deploy their mods and a server can use and sync the needed mods with the client once.

But okay, if you have programming knowledge it should be no problem to start with lua or anyother language. If you are new to programming anyway then i recommend you to get a big cup of coffee. Maybe then you should start with an C/C++ or C# or Java Tutorial series on Youtube. TheNewBoston has good lessons for beginners.