My dad worked for Pat Transit for 25 years. The subway is great, if you get a chance to ride it, since you are going to be downtown ride it to station square and then back again. If you want to see the 'fountain' that is in all of the pittsburgh photos ride the trolley (subway) to gateway center (the last stop on the line) and point state park is right there to your left when you get to the street level.
as for a bus this was on pitts website:
Port Authority Transit System (PAT bus)
PAT is a publicly owned mass transit system operated by Port Authority of Allegheny County. Fares vary within a five-zone system extending in all directions from downtown to the outer suburbs. Basic fare is $1.75. Exact change is required. Transfers can be purchase for a quarter, and can be used for one connection in any direction within three hours of the time of purchase; pay as you enter on inbound trips toward downtown Pittsburgh, pay as you exit on outbound trips before 7:00 p.m. After 7:00 p.m., pay as you enter on all trips. Local buses numbered 71 (1, B, C, or D), 67 (A or F), or 61 (A, B, or C) come into Oakland and may be boarded along Fifth Avenue, downtown. For bus routes and times call (412)442-2000.