Okay, so, First, you don't need DMG for now. But you'll need to do what I'm about to say. I think I'm about to give a good advice.
I'll repeat a few things anons already said but bear with me.
1. Read Player's Handbook. Read the rules. Memorise core mechanic. When in doubt about something, roll d20 + appropriate ability + proficiency if applicable vs DC. If you don't remember some specific rule, rely on that.
2. Prepare your game.
You can start with published modules or do something yourself but the important bit is:
Your first game should be short.
Why? Because it's going to be bad. And it's okay. That's how that always goes. It's going to be fun still. Think about like 3-4 sessions with strong obvious conclusion.
3. If you want to use published modules, go with the 5e Starter Set. It can be a bit daunting to you, but it's a solid adventure alright. But..
4. ...If you don't want to use published modules, it's okay. Trust me, they're not very good examples of how to actually make an adventure. They can be inspiring but it's always someone else's idea. Ad you can deal with your ideas in the way that suits you. But I understand, it's confusing. Where do you start?
Well, first of all, make it simnple and explicit. "So you guys are about to explore Snow Caves, since nearby mining town's council is bothered that its denizens are about to intervene with mining business. Or whatever. You'll get a reward and probably some nice treasure!"
Dungeon is a good structure for an adventure. Check this basic model of doing them, which is all you need for an evening worth of play: roleplayingtips.com/readissue.php?number=156#1
5. Once you figured this out, here's another stuff to remember:
- Combat is a big part of D&D, but it's not the best part. Not every encounter should be a combat. It's actually a good idea to put some powerful but not inherently hostile monster in the dungeon (cont.)
Try to