Remember that the idea will expand, and become more ambitious as you go.
Companies that I've been very involved with, that have had a very bad first hire in the first 3 or so employees never recover from it.
It really is true that you become an average of the people you spend the most time with.
In general, it's best if you're building something that you yourself need.
Be suspicious of any work that is not building product or getting customers.
For early employees you want people that have somewhat of a risk-taking attitude.