Depends where you are as to what's going to be on the market, but
Beginner Bikes does a reasonably good job of covering off the US market, and is free of the stupid advice that would see a newbie on 600cc+ bikes.
A good 250 or maybe, at most, a low-end 400-500 is appropriate. Ignore suggestions for the likes of the SV650. Too much bike for your first one.