You have played a game or two of D&D using the free resources and decided you like the game and want to get deeper into the WotC 5th Edition rabbit hole.
The obvious place to go next is to get one of the two starter sets or the essentials kit. But which?
You have to decide you want to create my own characters or not? If the answer is yes then the you go for the essentials kit. Otherwise you need to decide which starter set to choose.
Both of the Starter sets are good. Both have similar contents (basic rules, set of dice, pregen characters, adventure book). So it’s down to the adventure and availability.
Obviously the older of the two sets will be (eventually) the harder to get. However the included adventure Lost Mines of Phandelver is pretty good, and later this year WotC are publishing an adventure book that follows on from it.
The new starter set also has a pretty good adventure called Dragons of Stormwreck Isle. The advantage of this set is it’s a bit more DM friendly.
If you went with the essentials kit you get basic rules that cover creating rules, set of dice, DM screen, blank character sheets, condition cards, map, sidekick cards, item cards, combat reference cards, and the adventure Dragon of icespire peak.
After you have finished whichever of the kits you chose you will need the D&D 5e holy trinity of the Players Handbook (PHB), Dungeon Masters Guide (DMG), and Monster Manual (MM).
After the holy trinity your options are unlimited. You can run published adventures official or third party. Or create your own.
However I would recommend the Lazy DM series as a supplement to the DMG. I look on these books as the stuff the DMG forgot to tell you.
I’d start with the Return of the Lazy DM. This introduces you to a way of prepping your session that I really like. It does cut down on the amount of time required to prep. You can read about the steps in the Lazy GM’s Resource Document (which is a great resource).
The Lazy DM Workbook is a great book to have to hand whilst running a session. It’s full of useful tables, and ten generic maps.
The Lazy DM Companion has great advice in it for running D&D, plus pages of adventure generators, and maps. This is an amazing resource for planning your session.
Finally Forge of Foes is all about monsters. It’s only pre-order at the mo, with physical and pdf available later this year. Kickstarter backers have early access to the pdf. Not sure if a pre-order also gets it. But this allows you to create your own monsters, improvise monsters at the table, has advice about running monsters. It’s the missing sections from the MM.
All four have sample pdfs available that are useful without needing the rest of the book. Mike Shae has demonstrated this on his YouTube channel.
Also the resource document I linked to above has some of the content from the books too.