There are many great answers on this thread.
Let me just summarize my #1 observation here: the top mistake first-time sales managers make is recruiting. In that — they can’t do it.
It is incredibly hard to figure out how to replicate the magic of the first 1 or 2 sales reps that work closely with the CEO. And add in a first-time sales manager who may have limited experience recruiting a team, and it gets risky.
Generally, I would pass on hiring a sales leader who has never recruited at least a few reps that hit quota. This is very different than managing them, or inheriting a team. Going out and sourcing, closing and managing several reps to exceed quota.
If your first head of sales / VP of sales can’t bring in at least 1–2 great reps in her first 60 days … it’s usually close to hopeless.