To install a graphical user interface (GUI) on CentOS, you can choose from different desktop environments. The two most popular options are GNOME and KDE. Here's how to install them:

1. GNOME Desktop Environment:
- Open a terminal window.
- Run the following command to install GNOME:
```
sudo yum groupinstall "GNOME Desktop" "Graphical Administration Tools"
```
- During the installation process, you will be prompted to set the default target to graphical. Press `y` to confirm the change.
- Once the installation is complete, reboot your system:
```
sudo reboot
```
- After the reboot, GNOME should start automatically.

2. KDE Plasma Desktop Environment:
- Open a terminal window.
- Run the following command to install KDE:
```
sudo yum groupinstall "KDE Plasma Workspaces" "Graphical Administration Tools"
```
- During the installation process, you will be Press `y` to confirm the change.
- Once the installation is complete, reboot your system:
```
sudo reboot
```
- After the reboot, KDE should start automatically.

After the installation and reboot, you should have a graphical user interface available on your CentOS system.