Dming is not about authority or power, this will come naturally later.
Your very first task is to get the trust of the group. they need to believe that your main objective at the table is to give everyone a good time. You need to believe that too. Otherwise, your game will crash and burn, that's a guarantee.
A few ways to establish trust:
- Explain the rules to the players in little chunk, leaving unimportant details out untill needed. Helps maintain pace.
- Encourage players to make the charcaters they want with the Charcater Builder (free untill level 3) without you butting in (Don't worry, nothing in the core books and powers books will break your game... except the Monster Races)
- Play your dice out in the open (until trust is established)
- Always find a way to say YES to whatever PCs try to do... request a dice roll if it's highly imporbably and set a hard but achievable DC... they'll love it.
- An easy fight is not a failure on your part... it builds player confidence and sets a good first impression.
- When you are unsure of a ruling, rule in favour of the players but mention that you will check after the game. Build Goodwill.
- Listen to player chatter ans if they mention that 'it would be cool if 'x' happen' or 'I bet that 'Y' is behind it" change your story in the spot and make it so!
Once trust is established (over 2-4 sessions I'd say) you will start having what I call True Authority because it will be given by the players based on that trust and not the traditional convention of the Dungeon Master.
And that is the sweetest authority because you are considered part of the team that is your playing group.