By car
Most people get to La Mirada by car. From the I-5/ Santa Ana FWY, take the Valley View exit. It dumps you off in the industrial outskirts of La Mirada. Follow the signs that direct you North toward Biola University which is near the city center.
From the 57 FWY in Brea, exit Imperial HWY and head West. From the 60 FWY, take the Azusa exit South toward Colima, turn right on Colima headed West and though this is a longer route, it's hilly and scenic at times especially at night when the city lights are all aglow. Plus, it's a straight shot: Colima forks off into La Mirada Blvd after you pass Whittier Blvd and takes you right into LM.
By bus
You can also take the bus. Coming from Downtown LA, Montebello, and surrounding areas take the Montebello Bus Line #50. Coming from Orange County take the OCTA bus, either #20 from Yorba Linda or #29 from Huntington Beach. For maps and more details check out the city's map of local bus routes.
Nearby Norwalk and Fullerton have Metrolink train stations which have connecting bus routes and accommodate travelling further distances which can be painfully slow by bus. Check the 91 Line or the Orange County Line schedule.