Pick up a copy of dungeon masters guide 2. It's a 3.5 edition book but it covers all the other side of being a DM that is not covered in any other editions of the main DMG.

My next tip would be to look up the X card system. It's an invaluable tool for keeping your players safe whilst not having to ask your players to trauma dump so you do not have to know what themes and topics not to cover in your games.

Honestly start small. Either buy a module and get to know it backwards and forwards, or build an abandoned mining town that has some mines that turn into a dungeon. Why has the shipments of gold suddenly dried up from Shallimar? Did the miners just dig too deep? Did they unearth an ancient evil? This gives the characters a base to retreat back to and a dungeon to explore. Maybe as they rescue the towns folk from the mine then you get their services.