You have two choices with the factory headunit. Both involve tapping into the speaker outputs off of it; there aren't any built-in line outputs.
The first option is if your amp has speaker-level inputs. If it does, just connect those to the speaker outputs from your headunit and you're good to go. (Most amps that have speaker-level inputs also feature automatic on/off, so no remote turn-on lead is necessary -- check your amp to be sure.)
The second option is if your amp only offers line-level inputs, or if the speaker-level inputs are really crappy quality. In that case, you'll need to buy a line-out converter (LOC), and connect that to the speaker outputs from the headunit. This will give you a pair of RCA jacks at line level that you can connect your amp to. You'll also need to run a remote turn-on lead to the amp, and tie it into a switched power wire behind the headunit.
If you plan to eventually swap out the factory headunit for an aftermarket one that has line outputs, your best bet is the second option -- that way, you'll already have run the RCA cables you need, so the new headunit will be plug and play without having to run new wires to your amp.
