The only really practical way to get to this resort town is to book a flight into the airport at Zihuatenejo. Many U.S. carriers serve the airport directly -- especially from the midwest and west coasts. Many flights also originate each day from Mexico City. It's a small, clean airport with an excellent duty free shop!
To get to Ixtapa from, say Zijuatanejo, the easiest and cheapest way is taking the bus. It only costs a few pesos and it's just one bus.
To get around Ixtapa, I suggest walking, it's not a huge city and since you'll be on the beach doing nothing most of the time, you'll probably enjoy the exercise.