On the IT-201(2008) resident income tax return, line 69, where do I find the amount for
NY City school tax credit?

See the instructions for form IT-201, page 88. If you are single with income < $250K, your credit is $145. If you are Married Filing Jointly with income < $250K, it's $290. This assumes you were a full-year resident of NYC -- if you were a part year resident, see the instructions.

IT-201 Resident Income Tax Return (long form) Information