The game consist of two routes with 6 different endings, one of them being the true ending. On your first play through, i recommend going blind and unlock whatever ending you get, luckily, getting all endings is way easier than in SG1, since you dont have to activate as many "flags" (If you are worried about the achievements, you will be getting most of them but 1 or 2 by the time you unlock all endings).

So dont worry about the Rine messages (unless you go for real ending, read the real ending section first) or the random phone calls since the game sometimes picks them up automatically. The Amadeus calls are the ones you should be focusing on.

The real ending is pretty straight forward. You can unlock it after getting any of the endings, so if you want to get it asap, you can avoid all of the other endings.

                                                   Gehenna’s Stigma (Bad Ending)
[Closed Epigraph] (12/15 Wed) Shrine – Don’t turn your phone off
[Pandora’s Box] (1/23 Sun) In front of Daru – Answer the phone call from Amadeus


                                                     Vega and Altair (Mayuri Ending)
[Closed Epigraph] (12/15 WED) Shrine – Don’t turn your phone off
[Pandora’s Box] (1/23 Sun) In front of Daru – Ignore the phone call from Amadeus


                                              Recursive Mother Goose (Kagari Ending)
[Closed Epigraph] (12/15 Wed) Shrine – Turn your phone off
[The Orbital Eclipse] (1/2 Sun) Lab – Answer the phone call from Amaedus
[Anitomic Dual] (1/15 Sat) Lab – Ignore the phone call


                                             The Promised Rinascimento (Kurisu Ending)
[Closed Epigraph] (12/15 Wed) Shrine – Turn your phone off
[The Orbital Eclipse] (1/2 Sun) Lab – Answer the phone call from Amadeus
[Anitomic Dual] (1/15 Sat) Lab – Answer the phone call right away


                                                       Twin Automata (Maho Ending)
[Closed Epigraph] (12/15 Wed) Shrine – Turn your phone off
[The Orbital Eclipse] (1/2 Sun) Lab – Ignore the phone call from Amadeus



After you get your first ending, you will unlock the extras section, which shows which endings you have unlocked.
I recommend saving after you get any of the endings, the game saves automatically when you get redirected to the main menu, if you dont get "overwriting" notice for whatever reason, just load up a file and manually save.

                                                         Milky-way Crossing
1: Unlock The Promised Rinascimento ending

2: Either start a new game or reload a file from before the Closed Epigraph episode since theres
    a Rine message after the shrine scene that triggers the ending.

3: Continue playing the game by following the guide for Vega and Altair ending and finish the game     getting this ending (the true ending will unlock after the credits are over, so dont skip them)
 

So thats it! I hope you enjoyed the game. SG0 is a bit shorter and simple than SG but that doesnt take away how good this game is. If you have any questions, dont hesitate on dropping them in the comment section. Thanks!