Welcome MachinaMandala,
the first thing you’ll need is the rules, you can either get a mini version with just the rules or the full size book which includes hobby advice, profiles and a number of scenarios (mainly a campaign covering the first Hobbit film). You can also download the profiles from
https://www.games-workshop.com/en-GB/Ru ... bit-errata as
Heroes and Villains of Middle-earth.
The Escape from Goblin Town box (
https://www.games-workshop.com/en-GB/Th ... dition-ENG) is a great value set containing Thorin’s Company and some Goblin Town Goblins (including the Goblin King).
If you're definitely set on Isengard, Moria or Elves and not too interested in the scenarios it could be worth trying to find the mini version of the rules from eBay. You'll need the book for your force, either The Fallen Realms (
https://www.games-workshop.com/en-GB/Th ... -Realms-EN) for Isengard, Moria and Angmar (
https://www.games-workshop.com/en-GB/Th ... -Angmar-EN) for Moria or The Free Peoples (
https://www.games-workshop.com/en-GB/Th ... Peoples-EN) for Elves. Although if you were thinking of doing the new Thranduil's Halls you'd need There and Back Again (
https://www.games-workshop.com/en-GB/Th ... Back-Again) instead.
Hope that helps.
Once you have an idea about which force you'd like to collect come back for some advice about starting models.