Here's the Metrorail page. You would likely pick it up at the Douglas or El Segundo stations. Looks like Green Line to Blue Line. Not sure which stop to exit for the Coliseum.

Metrorail

And this from the Coliseum Website LA Coliseum

Public Transportation
Los Angeles Department of Transportation (LADOT) - DASH
Line F Weekdays through Downtown area to Exposition Park and Coliseum: Click Here and choose line F Weekend Information Click Here Southeast Line Weekday and Weekend Information Into and around Exposition Park, MLK/Figueroa area, and Coliseum (Saturday times shaded) Click Here and choose Southeast Line

MTA
Visit www.mta.net
Route 102 runs to the Coliseum
Click HERE for a direct link and choose Route 102 to view PDF file (includes weekday and weekend scheduling)

Other Resources
Transtar - Shows you where and how to take public transit between any two locations in Southern California. Page furnished by Southern California Association of Governments. Click HERE and enter your addresses