This game is extremely confusing at first, and the tips/hints given by the game are misleading, which leads to further confusion.
I spent a few hours banging my head against the wall before finally figuring things out.

The achievements are broken up into two halves: 'Dark Ending' and 'Light Ending'. The Dark Ending is easy: You just need to fail a bunch, which you will no doubt do as you learn the game. The Light Ending requires a bit of effort, but far, *far* less than the game makes out.

As mentioned these are pretty easy to get. You just need to fail the game a bunch of times (16 times to get all 3). The quickest way to do this is to just do nothing productive and sleep very little. Click on the desktop in your room, click the icon on the far left, and click the panel that shows up. Wait a minute or two and the day will end and you should die. You might need a second day of this.

Once you die, take a postcard off the wall, and the game will reset. It also might crash, but it should still save progress that you died. You can tell if that postcard appears on your wall next time you start the game. Repeat until you have all postcards.

Die once.

Die 8 times.

Die 16 times.

While the game hints advise you to go out and run and talk to the same people every day to boost stats, ultimately you can beat the game and unlock the remaining achievements with just the applications outlined below


Basically as soon as you wake up:
6AM - 12PM: Join your Class
12PM - 8PM: Talk on Messenger
8PM: Sleep

If at any point Coding or Photoshop become Active (they bounce up and down on the screen), run them concurrently with whatever you're doing for as long as they're Active, otherwise you can just ignore them.

Live until the rainy day (if you go outside it will be raining, but we won't be so shrugs).
Finish the game.
Finish the game without ever talking to someone.