Most probably you will need this:
https://www.games-workshop.com/en-GB/The-Generals-Compendium-Gamer-Collection-2017-ENG
It's got:
1. Narrative and open play helping points
2. Multitude of battleplans
3. Points, artfacts and battalions (different organisations of units that give you benefits) for all races
Mind you that I manage to do perfectly fine without it, but you'll probably want to get hold of a copy. The new mission cards are a nice addition for generating random missions.
After that you'd want a start collecting box
https://www.games-workshop.com/en-GB/Warhammer?N=647457687+219983815&Nr=AND(sku.siteId:GB_gw,product.locale:en_GB_gw)
or a starter set
https://www.games-workshop.com/en-GB/Warhammer?N=647457687+2528145041&Nr=AND(sku.siteId:GB_gw,product.locale:en_GB_gw) (a new double box is coming out next week - stormcast vs nurgle)
(or just the models you like). You can branch anywhere from there on. Each unit has free rules on its page and it's really easy to pick a new purchase based on the combination of gameplay and looks.
Skirmish is another mode of play. It uses fewer models (less than 10 usually) and has a separate book to help you out:
https://www.games-workshop.com/en-GB/searchResults?qty=com.gamesworkshop.endeca.EndecaUserContext%4047c62820&sorting=&view=&Ntt=skirmish
Also if you want to run a campaign starting with minimum models and collect an army as you go you can try out Path to glory. It's got random rules for generating an army and then running a mini campaign.
https://www.games-workshop.com/en-GB/AoS-path-to-glory-2017-ENG