You can certainly start looking for an agent now! Many agents will spend time with you walking through your home and giving you suggestion on preparing it for listing. In order to find someone with a proven track record, I would ask your neighbors and drive around your area looking to see who seems to be dominating the market there. An agent who has many listings will get many calls from buyers looking in that area, and all those phone calls will benefit you! You will be guaranteed exposure from that you would would not get from an agent with no listings in the area...
Also - I would do a search online for "South Side Pittsburgh Real Estate", or something to that effect online and see whose name pops up frequently. Chances are, that will be reflective of how much internet exposure your home will get which is extremely important as 90% of buyers begin their Home Search online!
Once you find a few agents, you can certainly interview them as would would for any job! Ask them questions about their plans for marketing your home, and what special promotions are offered through their broker that would benefit you.
Good Luck and please let me know if I can be of any more help!!