Now there are 4 stages of evolution starting from 5 Stars Digimon.

"5 Stars"
Child +10 = Adult
Adult +10 = Pefect
Perfect +10 = Ultimate
Ultimate (Last Stage)

"6 Stars"
Adult +10 = Perfect
Pefect +10 = Ultimate
Ultimate +10 = Super Ultimate
Super Ultimate (Last Stage)

"7 Stars"
Perfect +10 = Ultimate
Ultimate + 10 = Super Ultimate
Super Ultimate +10 = Mega
Mega (Last Stage)

You need to "Evolve" the first stage by using another same rank Digimon as catalyst to reach +10, So for example :

Agumon (5Stars) + Agumon (5Stars) = Agumon (5Stars) +1
Agumon (5Stars) +1 + Agumon (5Stars) = Agumon (5Stars) +2

Same goes with with other higher ranks.

And most importantly once you reached the +10 Evolution Stage you will need an Evolving Material and they're ridiculously rare and expensive and you need lots of them.

1st Stage = None
2nd Stage = Evolution Stones
3rd Stage = Life Stones
4th Stage = Moon Stones

First thing you need to know are these :

7 Stars Digimon (Yellow)
6 Stars Digimon (Red)
5 Stars Digimon (Orange)
4 Stars Digimon (Purple)
3 Stars Digimon (Blue)
2 Stars Digimon (Green)
1 Star Digimon (Gray)

Okay now you know them, next thing you need to know is that 5 of pieces 1 Star Digimon will make a 1 piece 2 Star Digimon through Synthesis. You know how to add right? So I believe this answers your question.

It'll need a lot of effort and dedication to get a 7 Stars Digimon if you are a casual gamer, or if you don't want to spend real money for the game.