[5e] [DM] Starting a campaign - looking for advice

I'm going to be starting campaign in the next month with one experienced player and three new players.

I'm new to DMing D&D but have GM experience in other tabletop games.

Should I run an premade adventure because of the new players or would a custom campaign work with appropriate encounters geared toward teaching the new players?

With previous games the party mutually decided to know each other but how could I approach it if the players choose for their characters to not know each other? I'm going to send out hand written invitations from the first NPC that is requesting their services but not sure how to approach if the characters don't know each other. Trying to avoid awkward moments if possible.

Thanks in advance.