While Google Docs doesn't have a built in citation generator, you can make your life a lot easier by adding one.
My favourite is the EasyBib Bibliography Creator, which is free and easy to use.
In the top menu, click on Add-ons. Then click on Get add-ons.
EasyBib Bibliography Creator will probably be one of the first options you see, but if not, you can search for it in the search bar on the top right.
After you've installed the app, here's how you can use it.
When you want to create a bibliography entry, again click on Add-ons. There, you'll see EasyBib Bibliography Creator. Click on it, and then select Manage Bibliography.
From the drop down menu, select the citation style you're using, e.g. MLA 8th edition.
Click on Book, Journal Article, or Website, depending on the type of source you are using.
For books it is always best to search by the ISBN number. Usually, you can find this number as part of the publisher bar code, on the back cover of the book, or on the back of the title page, inside the book. Searching by ISBN guarantees that you'll find information for the specific edition of the book that you're using.
Search for websites using the URL.
Journal / magazine articles can be harder to find, but many articles in scholarly journals published electronically (the ones you'll find in databases like EBSCO) have something called a DOI (digital object identifier). If you have this number, use it. If not you can search by title, author, or keywords.
When you've found the correct citation, click Add Bibliography to Doc. This will create a Works Cited page at the end of your document.
*Note* You should always check to make sure that your citation has been formatted correctly. Sometimes citation generators don't get it exactly right... punctuation may be wrong, etc...