Provided you have the latest nav-data (a self-installer you can get here: http://www.navdata.at/php/airac/airac.php), and pretty much even if you dont, you can go to the "Dep/Arr" page and drop in the runway you're heading for. Even if you don't understand all the options under there (i.e., the STAR, the fix that you want to intercept the approach at, which approach you're selecting, etc) just select something and don't worry. You'll get vectored in for landing so that stuff won't really matter to you if you're flying off line or under ATC control.
What you wanted from that is to tell the flight computer what runway you're landing at, so now when you go back to the "Init Ref" page (top left button on the FMC keypad), you should see the ILS frequency and course sitting right in the display, just below your approach flap/speed settings.
Simply dial that course into the MCP's course display, and then tune the nav1 radios to that frequency, and you're good to go! Don't forget once you've got the APP mode activated to hit autopilot command B.