So you have managed to successfully install and boot Ubuntu. Congratulations!
If you installed Ubuntu 11.04 or later, you might come across the LightDM, a type of login screen, before the Ubuntu desktop screen appears. To login to your user account, click your user name and type your password. Now, press the enter/return key or click the arrow on the right side of the box where you typed your password. IfAuto Login is enabled, the desktop screen appears instead of the login screen.
If you installed Ubuntu GNOME or a version of Ubuntu older than 11.04, the first thing you will see when you boot your new operating system is the Gnome Display Manager (GDM), your login screen. In the box you should enter your username which is the name you chose during the installation process. After you have entered your name press the enter key. You will then have to enter your password which you also chose during the installation process. The password will appear as black circles so that nobody can see your password. After you have typed this press enter and voila, welcome to Ubuntu!