After arriving at Narita airport, go downstairs to where the train platforms are and look for this counter where you can pick up tickets for the Skyliner. You can also pick up tickets for the local train that stops at every station too. I think the ticket is about 1,000 yen cheaper but takes longer.
You can also get into Tokyo by using the Limousine Bus. Costs 3,000 yen to get into Shibuya. Here is a list of places where the bus will stop in central Tokyo. Typically takes between 75 and 125 mins to get to Shibuya. I only recommend this route if you really cant be bothered to carry luggage around the trains. The bus is costly and often gets stuck in traffic. We decided to take the bus once and will never do it again ever! The bus was stuck in traffic for about 3 hours and then we got off to take a taxi to get back home - the bus follows a particular route so it wont take shortcuts to avoid traffic.
Speaking of Taxi, if you pickup a standard taxi outside the airport, be prepared to pay 20,000 yen to get into central Tokyo. Will take 60 - 90 mins.
Alternatively you can get a chartered taxi service through Tokyo MK - price list lives here. This service will cost 17,000 yen to get into Shibuya. Tokyo MK also have a pickup service which they charge an extra 3,000 yen for. If you have parents that you want picked up then you may want to give this service a whirl.
The fastest route between Tokyo and Narita airport however is the helicopter service provided by Mori Building City Air Services. The ride will cost 37,500 yen per person and take a mere 30 mins. Timetable is available here. The helicopter departs right from the center of Tokyo in Akasaka.