As a GM, you'll need to record notes about your campaign. I'm going to assume that you're using a physical binder (as I recommend; more on that later), but obviously you can replace "sheet" with "file" or "note" or whatever works if you're using a digital solution.
- Session logs
- PC sheets
- Faction sheets
- NPC names
I print what amounts to a 30-row spreadsheet on each page, where each page corresponds to a month of the year and each row corresponds to a day of the month. I then record where the PCs travel and what they do, and what various factions are doing ("5th day: Kalarel recovers 3rd piece of rod"). I'll also use this to record deadlines and when things will happen in the future ("Day 25: Kalarel completes the rod").
This lets me be specific with my players when giving them quests ("You have to get this artifact to Dwimmermount within a month") and answer their questions precisely ("We've been adventuring for a while now. How long do we have until Kalarel completes the portal?" "About a week.").
I have a separate sheet for each session, where I record what happened (high level, 1-2 paragraphs per session), what the players said they did or didn't like about the session, and ideas for the next session.
I like to have one "PC stat sheet" and several "PC goal sheets."
The PC stat sheet lists the race, class, passive Perception, and Armor Class for each PC. Just one line per PC. This is really convenient for battles, so you can just glance down at this sheet to see if an attack hits.
I also have a sheet for each PC, recording any plans players have for their characters, things they want their characters to do, and so forth. I try to review these sheets between sessions, so I can think of things to challenge or improve each character.
I've already written about creating a useful faction, either friend or foe. As a reminder, it helps to know the following:
- Faction Type (bandit, religious cult, merchant group, army, etc.)
- Leader (personality traits)
- Name (of the faction and the leader)
- Current Actions
I record each faction on a sheet or two, and update them as the campaign progresses.
I also try to make sure I have at least 2 enemy factions and at least 1 friendly faction in play in any given campaign. That gives the PCs enemies to play off each other and an ally they can use if things get dire.
This is one of the secrets of GMing.
Write down dozens and dozens of NPC names. You can generate or find a bunch of these online:
- NPC Naming Aids lists many Medieval European names
- Chris Perkins' NPC Name List has a number of names that would fit well into a classic fantasy setting
- Behind the Name will generate a name from one of dozens of different real-world or fantastical settings
- Fantasy Name Generators has generators for dozens of fantasy races
- FIFA 18 Top Players lists many real-world names by country
Often, the players will ask to go places that you haven't planned for.
For those situations, I like to have a handful of maps, typically:
- A manor house
- An abandoned temple complex
- A classic dungeon
- A forest dwelling
Basically, think about the areas of your campaign world and what a typical location in each area would look like. Then, find a representative map.
You can do this by doing an image search for "rpg map (location type)", such as "rpg map manor house".
If you're looking for a site with a bunch of awesome pre-drawn maps, look no further than Dyson Logos's maps.
There's plenty more you can add to your binder, but this will give you a solid place to start.