(If this guide is useful to you, then please rate it upwards.)

Use the WASD keys to move.

Press "L" to use your cigarette lighter as a torch.
(However, it is not very bright.)


There is a wheelbarrow blocking the front door of the house.
Hold down "E" and move backwards to drag it out of the way.
Hit "E" again to release the wheelbarrow.



There are light switches in almost every room.
Hit "E" to activate them, as the light from your cigarette lighter is not bright enough.



Press "E" to interact with, open, or examine things.
This will open doors in the house, except for a few locked ones.



When you find a piece of furniture blocking a closet, hold down "E" and move backwards to drag it out of the way.
Hit "E" again to release the piece of furniture.

Update:
Magdalena originally included some achievements with the game; however, from the start, they were broken. I wrote this guide on 8 November 2016, but when I checked on 23 December 2016 to see if they had fixed the achievements, I instead discovered that they had been removed altogether.
This section of the guide has been left in place, just in case the achievements are fixed and returned to the game.

There are four achievements for Magdalena.
However, at the time of writing (8 November 2016) the achievements do not work.
The developer has been promising to get them up and running, since at least 10 April 2016.
At this stage, we are stilling waiting.

When the achievements are finally enabled, then this guide should hopefully cover everything.

1) Don't look back
You read a Slender's note.
-- Presumably Slender is the author of the notes.
There are three notes around the house.
a) Beside the front door (to the right).

b) On the table in the dining room.

c) At the right side of the house (outside).



2) Not a step back!
To go in opposite direction at the start of the game.
-- This achievement seems easy enough to do.
When you start the game, just turn around and head off in the opposite direction.



3) Secret room
You are trapped in the secret room.
-- Possibly this refers to the walk-in closet in the main bedroom (upstairs). You have to drag the chair out of the way, in order to access the closet.
We will have to wait for the achievements to be enabled so that it can be checked as to whether the closet is the right "room" for this achievement.




4) What's behind the door?
You opened the children's bedroom.
-- Presumably this is the room at the end of the upstairs corridor, i.e. the room with the toy monkey in it.



A more definite guide to the achievements can be done when the achievements are actually working.

Early on, the achievements had an attainment rate of 2.3% to 2.4%, but I have to wonder if that might have been the result of the developers testing the achievements.
https://steamhost.cn/steamcommunity_com/stats/432470/achievements/

There are some videos (not mine) on YouTube which showcase the game:

https://www.youtube.com/watch?v=u6JgA3ln6E8

https://www.youtube.com/watch?v=mnhdEbJMW7Q