what's the first thing i need to check when my 2002 mr2 spyder "check engine" light just came on?

Have the computer scanned at your local parts store for FREE. They will be able to tell you what the CEL is indicating.

make sure your gas cap is securely on-loose cap shows as a leak in the evaporative rec overy system - most common cause is loose cap or bad gasket on gas cap

Oxygen sensor malfunction is a common issue, exhaust side ( right side, facing front of car from rear ). You will need a deep socket to extract the sensor and can buy these for $45 - $50 on ebay, $99 at AutoZone. I use Bosch sensors only.

I have the same issue with a 2002 MR2 that i JUST bought. Check engine light came on after driving for 20 minutes. Code PO155 which indicates 02 sensor. Is this easily replaced? Are there youtube videos or other instructions on how to do this repair? thanks

A cheap fix for me was to put Seaform in the gas tank. A lot of times you have old gas in the tank. This solved my problem however its a good idea for any auto this age. Made a night and day difference on my 97 Dodge Ram during starts