You didn't say where in San Fernando valley but
go to: http://www.metro.net/index.asp
and enter LAX as your origin and the actual destination and you'll get some options. Cheapest would be Culver city 6 to Westwood connecting to MTA 761 or Santa Monica 3 connecting to MTA 761 each for $1.25 one way but taking 2 or 3 hours. For $7 one way you can take the Van Nuys FlyAway bus to the valley and then the MTA bus of your choice to your final destination. Fly Away only takes credit cards now and picks up right outside the terminals. The Culver city and Santa Monica bus to MTA connections require you to take the Parking shuttle to Lot C and get off at the lot entrance where the bus terminal is adjacent.