Always refresh the local package index before installing software:

sudo apt update

Install a package:

sudo apt install nginx

Upgrade all installed packages to their latest versions:

sudo apt upgrade

Remove a package (keep config files) or purge it completely:

sudo apt remove nginx          # uninstall but keep config
sudo apt purge nginx           # uninstall and remove config
sudo apt autoremove            # clean up orphaned dependencies

Search for a package and inspect it before installing:

apt search web server
apt show nginx

To list all installed packages or check whether a specific one is installed:

apt list --installed
dpkg -l | grep nginx