If this is a 4 cylinder replacing the plugs is easy, the Chiltons manual has instructions or look it up on line (1999 Toyota Camry Spark Plug Removal and Replacement - YouTube). Remove the plastic engine cover if it has one and remove the coil covers. Use compressed air to blow out any debris. Due to the age it would be worth spraying PB Blaster on each plug and let it sit for a while. Might stink when it burns off, but reduces risk of messing up the threads. I found the only place to get the proper Denso plugs was the dealer and the price was not out of line