Question: first things to do after installing manjaro kde?
Answer: Install Pamac. Pamac is a graphical package management program that can be used instead of Octopi. ... 
Disable GRUB delay. By default when your Manjaro boots, you see this GRUB menu. ... 
Reduce swappiness. ... 
Install Firewall. ... 
Extend spell checking. ... 
Install MS Fonts. ... 
Enable TRIM for SSD. ... 
Remove Orphans (Unused) packages.