Welcome! I'm a new player myself, just started collecting an army this year and gotten to play a few games. Right now the only starter set is the Escape From Goblin Town which has the small version of the rulebook as well as a great start to a Goblin Town army as well as Thorin's Company and Radagast. While technically Thorin's Company can be fielded as an army in points match games, most people don't. They can be used to lead troops from the Iron Hills faction though.
I think I may be getting ahead of myself, first it would be good to know what kind of games are you looking to play? Do you want to recreate events from the movies and books in scenario play? Or play against each other in points match games?
If no one in your group likes the idea of playing as Goblin Town or using the Thorin's Company models you can buy a bigger hardcover version of the rules seperately.
https://www.games-workshop.com/en-US/Th ... Journey-ENHowever there is a new rulebook expected to come out Summer 2018, so you might want to hold off splashing cash on the $85 book from GW. You can find the Escape from Goblin Town rules on ebay or trade forums from time to time.
After the basic rules you will need the profile book that has the faction you want to play (again this is assuming you want to play points match games). The Desolation of Smaug book has all the heroes, villains and armies from the Hobbit movies you will need.
https://www.games-workshop.com/en-US/Th ... Back-AgainIf you're interested in playing factions from the Lord of the Rings movies you'll need one of these books:
https://www.games-workshop.com/en-US/Th ... 0580000%5DMordor has everything you need for Mordor.
Moria & Angmar for Moria and Angmar.
Fallen Realms is for Isengard, Harad and Umbar, Easterlings and Rhun
Kingdoms of Men: Gondor, Rohan and Arnor
Free Peoples: Rivendell, Lothlorien and Mirkwood (not to be confused with the Thranduil's Halls profiles from the Hobbit movies), Durin's Folk, the Shire and some others.
Again, these profiles might change with the new edition of the rules, nobody really knows what that will look like. For sure.