I'd love to provide you with this answer, and particularly with details. You see, there are two popular and equally correct answers, and I'm prepared to offer both to you, with the understanding that the one which sounds easier is much much more dangerous to your data, while the one which sounds slightly more complicated is immensely safer, while also being easier. Given the choice I'm a big fan of both fast and easy. Neither method is significantly slower than the other.
Also please see my bio and take my word for it that the program I use for cloning is in fact the slightly more complicated sounding solution. So lets get started.
The solution I wish to recommend to you is CLONEZILLA
Clonezilla is a program which comes to you as a downloadable ISO which you burn to a cd and boot your computer to in order to operate it. After several text menus, it gives you the stupid easy choice to back up your drive (or just partitions) to an image file on external usb media (in most cases) or directly to another drive entirely. I recommend the image transition, if only because I'm fond of backups. Using an intermediary takes longer, but I think it is worth it.
The other solution uses a utility built into Ubuntu called dd. I have heard that dd official stands for data description, although it is commonly referred to as "drive duplicator" but that it's ambiguities which causes people to copy blank drives over full drivers resulting in Data Destruction.