You'll need to use the gravity gun a lot in Ravenholm to conserve ammo there are an enormous amount of zombies, but also lots of handy things you can fire at them, too.

There's a zombie out here use the gravity gun to grab an explosive barrel, then put a bit of distance between you and it and fire the barrel to take care of the zombie. Your destination is the house at the back on the left the one with the entrances covered up with boards.

As you can see from the outset, things have not gone on well in Ravenholm - the headcrab shell and legs hanging from a rope tied to the tree prove it.