okay today i took my 99 GS300 to a mechanic and did complete tune up and complete timing belt change, including most seals, water pump, thermostat etc.... now my problem is when I drove my car home the engine light came on (code: P0171 ) it says system too lean. Now what should I do?

Maybe the mechanic left something unplugged/unhooked? If you find something to plug back in, you will have to reset the code by unhooking your negative battery cable for a couple minutes. You will lose your radio presets.

If you can't find anything simple, I would take it right back to the mechanic and ask why did that happen right after he worked on it. I would give him a chance to take a look before jumping too hard on him, there might be a reasonable explanation?

As Hillsndale suggests, if this happened right after the service, have your mechanic check all connections that might not be secure, sensors, vacuum lines etc.

isn't a P0171 a o2 code? check bank 1 o2 sensor from my recollection