Some tips for newbies in the game:
When you get the maximum evolution, feed your monsters until they're full
Try new teams. A team composed by the same old monsters might get your game a little boring
Try getting S-rank on all mission before proceeding to the next missions
If you're low on money, do the random mission. They're quite nice and give you a good amount of cash
Try getting all evolutions, so you can test all teams and find the one that you most like
(Ex: My endless mode team is: 2 Shadow Dragon, Entropy Alien and Corpse King)
That's about it, but the most important advice is: HAVE FUN!