Take a bus from airport to termini it costs 5 euro.then take bis 105 to parco di centrocelle. This is the easiest way if you hqve never been to Rome. Bus no 105 is right in front of termini station and the sign is smal you need to look carefully. You can not buy bus ticket on the bus. Yiu have to by it in any news agency or tobacco shop. There are many aroubd termini station. When you know the city better.you will find many other ways to get there This tip is very useful