D&D is free. You can play while paying ZERO money*.
FREE BASIC RULES
You can get 95% of the character rules, spells, magic items, and monsters for free using the basic rules PDF from Wizards of the Coast (the company that makes D&D, sometimes called WotC).
This is the try-before-you-buy versions of the Players Handbook, the Dungeon Masters Guide, and the Monster Manual all rolled into one. If you're new to D&D or new to 5th Edition, this is your way to try everything FOR FREE.
SYSTEMS REFERENCE DOCUMENT
You can also find the Systems Reference Document (SRD), which is the same information as above, here:
The SRD is designed to have all of the basic rules so that other game designers can take the 5E framework and make their own classes, magic items, monsters, and rules and build games based on 5E.
CHARACTER SHEETS & CHARACTERS
Here are some free character sheets from WotC and their versions of 16 different characters with character sheets for levels 1-10 and a bunch of different character portraits for inspiration.
THE DMs GUILD
The DMs guild is a website (run by DriveThrough RPG and Wizards of the Coast) where ANYONE can self-publish material for 5E/Forgotten Realms. You can find a variety of adventures from fun homebrew to gorgeous professionally produced epics - and generally for very low prices (most of it is free to $5). Here's what they have for free. You will need to make an account and "purchase" these for free.
FREE ADVENTURES
I’ve seen a lot of questions where new DMs are looking for good adventures to run. Lost Mine of Phandelver gets recommended a lot, however, it’s 15-25 hrs of content (or more if your group likes to explore and roleplay). That can be very overwhelming for a new DM. These are much shorter adventures that are a little easier to DM because they're less material.
Here are some things I recommend for new DMs that are free.
The first part of Storm King's Thunder (called "A Great Upheaval"). In this adventure, the heroes rescue a town from a goblin invasion and deal with the aftermath of that invasion. It starts characters at 1st level and takes them to about 5th level and is probably about 6-12 hours of content.
The first part of Curse of Strahd (called "Death House"). In this adventure, two kids on the side of the road ask you to take on the scary monster in the basement of their house. Absolutely perfect for a Halloween one-shot. It starts characters at 1st level and takes them to about 4th level and is about 4-8 hours of content.
Encounters on the Savage Seas III/Free is a pay-what-you-want set of six encounters and mini-adventures set on the coast or ocean. (I have an adventure in this book.) Since it's just encounters with no storyline to connect them, you can, as a new DM, just play one encounter to see how DMing works. These are for a variety of levels but are rarely more than an hour or two of content.
The Black Road. Ben and I wrote a top 100 bestseller on DMs Guild. If you want, I will send you a copy for free if you PM me your DMs Guild email address on Twitter or Facebook This simple adventure that's great for new DMs and new players. Exploration, combat, and RP encounters. It's written as a one-shot, but has lovely dangling plot threads for you to build on if you're interested in doing so. This is for characters level 1-4 and is about 2-4 hours of content.
Get out there and DM. You can do it - for free!
* Don't pirate stuff. The people who make the content should paid for it.