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This is for those who want to get all the posters as well as finding the juicy secrets that give decent items in the first level.
I’m making this guide since I haven’t seen any other guides, so this will be a rough outline on what you should do.
Poster #1: The Shovel
At the beginning of the game, leave your room which would prompt the game to lead you to a breakable door. Once you break the door, go pick up the shovel but make sure to turn around since the poster is right on the wall opposite it.
Secret #1: Dopefish
After completing the first tutorial, you should be out of the house but getting shot by some old veterans. Be sure to go to this brown house that is to the left in order to find your first secret.
Once you are inside, you will notice that the house is pretty much a ♥♥♥♥ house. There are 15 TOILETS that you must fill to the brim with your piss. 14 should be inside while 1 is outside close to the right of the house. 
Once the game says “The sequence is complete”, return inside the home and go to the garage room where you will see an opening in the floor with the Dopefish spray. You will be rewarded with a powerful rocket/holy grenade launcher.
Poster #2: Golden Retriever
Go back outside and you should see an orange house right across from you.
Go towards the backyard of the orange house, there is a break in the white fence and a couple of dogs guarding it.
Continue onwards and you should see the dog poster and a ramp back to the street.
Poster #3: Burger Boy
On the street, you should be able to see a Burger Truck with a flying fatso near it.
There is a poster right behind the truck.
Poster #4: Cat
Continue down the street until you see a dark pink where there are quite a few cats in front of it.
Enter the house to find a single cat lady with a lot of puss. Search until you find a garage room.
The cat poster will be on the wall. Feel free to pet the cats for an achievement.
Poster #5: Straight Outta Suburbia
Literally right across from the pink red house, there is an orange house.
Kindly trespass into the home of suburbanites, one of the rooms has this poster.
Secret #2: Lady Dimitrescu
Return onto the street and you should see a decrepit, wooden building in the middle of some wheat down the street. Watch out for a large dog by the way.
Enter the wooden to get the meat hook shotgun which will absolutely slay your inner demons.
You will be attacked by a dog and an old man, so go upstairs after you destroy them.
You should see a yellow key at the end of the upstairs area and some boxes.
After picking up the key, crouch down near the boxes and walk into them. You should see a sexy surprise. 
Poster #6: Golden Retriever but Meaner
Go back near the stairs where you see an opening and shotgun shells on platforms.
You should see the large branch on your left. Try to jump around the branch to get to the poster.
Poster #7: Not-So-Smart Pistol MK420
I will assume you went to use the yellow key, blew up the barrels, and just crossed onto the second area. At the start of the second area where you see the folks on the cliff edge, make a sharp right turn to see a dark cyan green house. Either enter through the house or go around it into the back.
The poster will be on the wall of the house along with some angry old dudes at the pool.
Poster #8: Veteran
Near the cyan house is an army green house that has the next poster.
It’s the house that is right next to the gallows (the wooden platform with swings).
Enter the house and one of the rooms should have this poster near the bed.
Secret #3: Liquid Nitrogen
In the exact same room that has Poster #8, open the closet to find a pink lever.
Interact with it and a hole will open up in the room.
Drop down to find a consumable item that turns your piss into liquid nitrogen that freezes enemies.
Secret #4: Purple Portal
Go down the street where you will see a Mr. Krotchy billboard.
Continue down the tunnel and you’ll see a blue house where the blue key is stored. Be sure to grab it unless you wish to do more walking.
There is a junkyard inhabited by a few dogs and an old man, a purple portal will be in the back.
Going into it will teleport you into a house.
Poster #9: Pastel People
The poster should be right in front of you when you walk into the previously mentioned portal.
Poster #10: Super Hooker Shotgun
Go back onto the second area and look for a black house that is near a cliff edge that overlooks the blue key house area.
You should be hearing heavy rock music, just go inside and a shotgun poster will be up for pickings. 
Secret #5: Hot Sauce
Okay, all you need to do is to use the blue key in the house that is going uphill. I will warn you that there will be a group of enemies that could prove challenging. Here you will learn how incredibly useful the hooking mechanic of the shotgun will be in the future.
Once you navigate through all the other stuff into the third area, you should see a yellow house near a cliff edge. 
Go to the grill that’s behind the purple umbrella and look down, but don’t go off yet. You should see a glowing Hot Sauce bottle on top of a toilet on a small island. You will need to slide jump off this cliff onto that small island to get this secret. I advise you to save at this place because if you mess up, you will have to walk all the way back here.