DnD 5e - Choosing a Dungeon Master
Once your in a group, unless the group already has a Dungeon Master, the group will need to select one.
If someone in the group volunteers, let them do it. Being the Dungeon Master is a challenge, but if someone feels ready for that challenge it's good to let them try their hand. If it doesn't work out, you can always change Dungeon Masters later.
If noone volunteers, your next best bet is a player who has played the game before. Even a few sessions of experience gives a great foundation for how the game is played and how the rules work. Expertise in the rules isn't required.
If no one in the gorup wants to DM, consider doing it on a rotating basis. One DM will run the game for a few sessions, then rotate out. Each person in the group can run a short adventure to try their hand, and the group can continue rotating until someone decides that they're ready to run a longer game.