I want to start scripting a "Defend the Statue" game from AI, Where do I start?

If you've never started scripting before, this is probably not where to start (yet). I'm not confident that I know what a "Defend the Statue" game is or what AI has to do with it (I'm guessing you mean NPCs that attack the statue?), but if you'd like to get started I strongly recommend using the wiki.

It may seem boring at first, but the Roblox Wiki provides exactly what you need to get started, learn how to write code, and how to see results.

A lot of the new tutorials on the official wiki are stupidly basic, but tutorials like these actually do have useful materials for beginners:

For absolute beginners- http://wiki.roblox.com/index.php?title=Intro_to_Scripting

How to instantiate objects and edit their properties- http://wiki.roblox.com/index.php?title=Creating_Parts_via_Code

A very basic game where the players have to defend the statue would include a number value representing the statue's health, or to make it easier you could use a humanoid for that. I'm guessing that you'd want the enemy NPCs to damage the statue and also the players, so it could be like a zombie wave where a new enemy is cloned into the workspace and it automatically wants to attack the statue by using WalkToPoint or WalkToPart.

You could probably make a basic version of a game like this once you learn how to do a few basic things, and the wiki contains everything you need to get started.