You need to save up a total of 80,000 points to buy the first upgrade from the cat. You can earn points by collecting coins and killing enemies. If you get to the cat and don't have enough points you can kill yourself and respawn at the last checkpoint and the coins and enemies will respawn and you can get more points. Be careful not to lose all of your lives or else you'll lose all of your points.
The cat can be found on Level 1, not to be confused with Level 0.
To get to level 1 you need to take the bottom exit on Level 0.
To make things easier you can enter the Cheat Code from the main menu using the
and give yourself God Mode and Fly.
The Cheat Code is U-U-D-D-U-U-R-L-L-R-Y
U=UP - D=DOWN - R=RIGHT - L=LEFT
I show all Achievements in my Walkthrough here.
Timestamps:
Intro 0:00
Cheat Code 0:40
Level 0 1:10
Insta Buy 7:30
Level 1 2:30