You will learn that the way to White Forest is being blocked by the Combine soldiers. Luckily, the rebels will show you a safe way to get to the barricade. Follow the soldier and jump into a hole in the floor.
You will get to a room with windows boarded up. Remove the boards. There will be zombies outside so shoot them from inside the room. The won't get to you.
Get outside and on a lorry to the right. Take the supplies and climb up the roof.
Remove the boards and get inside. Then walk to the stairs.
Get to a passage on the right. Remember to crouch.
Head towards a tunnel and watch out for crawling zombies and headcrabs.
Get to the red car. You will have to destroy the turrets that you can see shooting on the screenshot.
Use the grenades to blow up the turret next to the building shown on the screenshot (it's surrounded by black smoke). It's not easy, but not impossible either. And still, remember to crouch.
Head towards the building taking cover behind the cars.
Smash the window and get inside. There will be some soldiers. Kill them. Search the building and take any supplies. Then head towards a white van.
Walk through that van and watch out for headcrabs and zombies. Head left to another white car.
Walk through that car and kill a crawling zombie. Then turn left.
Get to a rusted car. Watch out for the soldiers!
Then keep going right.
You will get safely to the building with turrets from the right side. Get to a blue lorry. Mind the zombie behind the corner.
On the lorry there are some supplies. Take them and get on the roof.