Many genealogists, including those here at WikiTree prefer the Chicago Style:
Chicago - refers to The Chicago Manual of Style which is a style guide for American English. It is the style most often used by genealogists.
Mills, Elizabeth Shown. Evidence Explained: Citing History Sources from Artifacts to Cyberspace. Baltimore, Maryland: Genealogical Pub Co, 2007.
So that is Author, title, where published: publisher and date. This is great for a book, but not always possible with some sources.
Find A Grave provides a link you can use. It is located next to the Memorial number and called "view source." For your profile:
Find A Grave, database and images (https://www.findagrave.com : accessed 16 May 2018), memorial page for Thomas Milton Terrill (18 May 1850–22 Sep 1912), Find A Grave Memorial no. 40462748, citing Grand Junction Cemetery, Grand Junction, Greene County, Iowa, USA ; Maintained by 46620252 (contributor 46620252) .
It will look better in your profile. You can just copy and paste.
FamilySearch also provides a citation you can just copy and paste:
"Iowa, County Marriages, 1838-1934," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:KCWZ-YPC : 27 September 2017), Thomas M Terrill and Lydia M Hill, 06 Nov 1879, Jefferson, Greene, Iowa, United States; citing reference pg 459, county courthouses, Iowa; FHL microfilm 1,034,210.
Ancestry is a little more tricky because the whole point of providing sources is so that other people can look at what you found and make their own determination. Most people do not pay for Ancestry. I like to put the original source first, then add on the Ancestry information. It would look something like this:
Original Source; database and digital images (if there are images), “Washington, Select Death Certificates, 1907-1960,” Ancestry.com Operations, Inc. (http://www.ancestry.com : accessed 16 May 2018).
Then below it, I transcribe whatever information is useful. Sometimes you can just copy and paste from what Ancestry lists.
For your super cool family diaries, I would write it something like this:
Special Collections Department, Iowa State University, Ames Iowa; “Terrill, Thomas M. (1850-1912) Papers” (these papers including his all his annual diaries from age 21 until his death (1871-1912) are located here. The originals were donated to this archive. Copies of his diaries which were self published in 1983 by Lois Terrill Mueller, one of his granddaughters (she also donated the original diaries).
It would also be helpful to include in-line citations. That just means that you note the source for each fact in the biography to show how you came to that information such as birth date, occupation, etc. Also, you do not need to type in "added;" you can just delete the default info [like this was generated from a gedcom, etc.] and write an actual biography, like a story.