I don't know, I'm not a very good DM
But really, the game has to be a mix of seriousness and fun. If you're too serious, the game isn't enjoyable, but if you aren't serious enough, the game just goes way off track.
Be well prepared, and know the rules (or make cheat notes) for everything you intend to use. Nothing disrupts gameplay like flipping through rulebooks every turn (but believe me, it _will_ happen).
Encourage (ie, force) your players to do a backstory. Make them decide on their character's history, motivation, likes, dislikes, etc. Experienced players will generally do this on their own, to some degree, but new ones might need a guide. I highly recommend starting with
this list of questions and modifying it to suite your needs. Encourage the players to seriously think about their characters' motivations and personality, and to behave accordingly.