GUIDE if someone's confused:
Once you appear next to the train, you need to look for people in their hiding spots, the trash piles are breakable. Once you find humans, dont shoot at them, just collect them and they will head towards the train.
Try to not leave people till they're safe or they will be eaten. Monsters appear from the grass so you can control the location if you remember all their spawn spots. Monsters will try to kill humans and invade their hiding spots so you gotta hurry, i usually quickly save two nearly groups, guide them towards the train and then come back for the others.
When you kill the monster, it drops flower
, you can collect it to gain ult, which gives you more speed for a few seconds and heals your xp a bit, so try not to skip them.
WASD to walk, LMB to attack, RMB to ult
You can watch your health and ult bars in the top-left corner of the screen 
Now the characters:
Tractor uses guns, he is universal and easy to play
Raskal is close-quarter fighter with high speed and health, ult is important since it's hard to avoid damage for him, so just jump into the crowd and spam ult
Peri attacks with acid, he one-shoots everyone except yellow monster, you need ult for it. He is slower than the others and has less xp, but makes high damage.
Jaderka uses laser that can shoot through the walls and monsters, which is useful against big crowds, she is the fastest to destroy trash too
For first time expirience i would recommens Tractor or Jaderka