Welcome to SBW!!
This is a topic that comes up often.
There are lots of good first bike out there.
You did not say if you were aiming at new or used......
One of the most recommended around here is the NinjaEX500.Also the SuzukiSV650 is high on the list.The GS500e Suzuki is good,too.These are good new or used.
Used: VF500F Interceptor,Katana600,Bandit600,600RNinjaR,Yamaha6 00Seca,the list can go on.....
Allow money for insurance and a GOOD helmet and riding gear,etc,etc.
Take you time and read and ask lots of questions,too.
Also take the MSF course(they have an "800#" in most cycle mags) and join the AMA too.Both these will get ya discounts on your insurance rates!!
Be safe and be smart!!