Firstly, I'd recommend against this, unless you're a very advanced user. If you're just hoping to use Ubuntu because you're used to it, you should be fine "settling" for debian. They function almost identically, and you'll be able to use apt-get and almost all guides geared toward Ubuntu. This being said, if you REALLY want to use Ubuntu, and you're okay jumping through a lot of hoops, here's the link you're looking for.
https://developers.google.com/compute/docs/images#buildingimage
Again, I'd highly advise against it unless you know exactly what you're doing.