A good DM must be a good actor.
You must be a lot of characters and, unless you want to be only a referee of emotionless fights ("I attack" "You hit" "I attack" "You miss") you have to role them, like an actor does.
You're not running for an Oscar Award but you can have fun even if you're not very good in acting
Move your hands, move your feet, move your body. You will surely help your players to view correctly what's happening.
Usually I move showing some combat scenes or players' movements or sometimes (for example during dungeon explorations) I tell a player "stand up and show me exactly what your character's doing".
Use physical mannerism for NPCs. They're helpful for your players to remember them. In the current Out of the Abyss campaign, players love Buppido's exhuberance and they have known it since he uses to jump a lot of times with his hands high trying to catch their attentions or volunteering for a mission. They're also fond of Shushaar: the Kuo-Toa moves endlessy with sudden changes of directions just like a fish in a bowl.
Use you face to describe a situation, just like;
Player: "I'm trying to bribe the guard with 25 gp" (roll for the check)...
....(DM stays silent)...
Player: "Do I succeed?"
DM: "Did you look at me? Look again! Do you see my face? Well the guard is looking at you with the same expression!"
The player will must decide if he has failed the check or not.
If you can, stand up. You'll find it's easier.
You can do anything with your voice.
Change the tone or use verbal mannerism or use some typical quotes for your NPC.
The quote can be funny (in an old campaign of mine I had a recurrent NPC say "I must do it because I have X hungry sons" and everytime I changed their number) or serious:
For example in the Out of the Abyss campaign, the myconid Stool has said so many times with a voice of a little child "You promised you would take me home" that players have just changed their path to bring it at home!!
Use several accents and don't mind if you're not good at it. Your players will recognize your efforts and will be deeper involved in the game.
Many years ago I ran a Forgotten Realms campaign settled in the Moonsea area and I started to use the dialect of Rome (I'm from Milan) any time a Zhent (Zhentilar or not) was speaking. It was a great succes and if I'll ever be back with those players to Zhentil Keep, I'm sure they're expecting to hear again the same dialect.
Reach that point and you've reached your goal as a DM.