I have Asus g551 laptop, it has uefi so during startup it gives ROG (Asus brand for gamers) logo then it begins. I am wondering if I installed Ubuntu besides my windows. It would give me a choice of what os I enter?
Hardware: Asus G551 laptop
The Ubuntu installer will detect existing OS on your computer and offer an option to install Ubuntu alongside your existing OS.
When you select that option, it will automatically setup the boot manager for you so it will display a selection screen for what OS you want to boot when you turn on the computer.
To have a dual-boot Ubuntu/Windows configuration, the easy way is to install Windows first, and then install Ubuntu.
In any case, make sure you read the installation instructions for Ubuntu, your question and the necessary steps are covered in detail there.