You can take the Mears Shuttle, this is a shared shuttle, which is about $16 per person round trip. To use Mears you merely call them and make the reservation at least 2 days in advance. They will pick you up at the resort and take you to Universal. About an hour or so before you are ready to leave Universal you call them to make arrangements to be picked up.
You can also book a town car service, the cost would be about $80, plus tip. A cab will cost about $26 one way, and could be higher depending on traffic.
You can also rent a car for the day. We did this and it worked out great for us, this way we could arrive based on our schedule and then leave when we were ready.