After the scene, drag Helena along. Pull her up and continue to the door. Attempt to kick it open then pressing the X button again for another action. Time the reticule so you hit the lighted area in the action circle.
Continue to the next room then examine a soldier's corpse by the window. Continue outside then once in control again, have Leon look through the window of the crashed Humvee to find an herb inside. Get it follow the instructions to open up the menu, mix the herbs and put it in tablet form. Give it to Helena afterward.
A zombie will attack afterward. Attempt to shoot it but since you don't have enough ammo, you'll be forced in an action sequence to wrestle your way out of getting bitten. Follow the onscreen prompts.
Return to Helena again and give him the medicine. Continue forth to the next door and follow the corridor to the next door. Exit then kill the zombies below. Don't forget to collect the ammo along the way. Also, make sure to switch to your melee weapon when they're too close to shoot.
Continue to along the way until a scene triggers. Keep running until you reach the helicopter. During this sequence, keep an eye to the action buttons that will appear.
Running away from explosion
Once you're inside the building, open the door and vault over the railing. Continue forth to complete this sequence.