I will skip conversation pointers, as those choices are entirely up to you. (Therefor I will not post guides for chapters 2, 6, and 8, since there are only conversational choices to be made - plus some zombie battle in 2)
Once Kenny has climed up the ladder, you should try to climb it too. It will break.
Walk to the front of the jeep, and take the tow cable. Attach it to the truck axis (between the upper part of the wheels).
Walk back to the jeep. there's a winch on front part of the passenger's door. use it to make the jeep tow itself towards the truck.
Climb the jeep and have Kenny try to pull you up. It will fail, and your first 'big' choice of the game will be made.
Will you shoot the girl, or leave her? It's a pure morality choice, as it doesn't matter how many supplies you get from the drug store (the reason to let her live, as Kenny puts it, is so she can distract the walkers for you). Aiming might be a bit hard as it's the first time you have to do it in this game, but as long as your crosshairs are in the general direction of the girl's head you'll hit her.
After you did or did not shoot her, the achievement will unlock.
EDIT: Negative voters, please leave a comment on why you think this solution isn't good, so I have the chance to improve it.