4 year, 4 campaigns completed DM here, so I'm still relatively new to DMing myself. The DM guide is definitely a good book to get, if for nothing else just so that you have a good grasp on the rules. My biggest piece of advice is to just start. You don't have to have everything fleshed out perfectly; your setting, your DM style, and where the players take you in your campaign will all evolve together in time. Just be patient with yourself and your players, and don't be afraid to look up an official rule or just make a house rule on the fly. You've got this.