The informal word for telling on people is "dobbing" (and we call that person a "dibber-dobber", at least here in Australia.
Is there an actual word for it?
Example: Sue went to my mother and told on me when I said that I was having an abortion. Sue is such a [insert word].
Inform (on somebody) — M-W
verb to give information (as of another's wrongdoing) to an authority
"informed on a member of his own gang"
Related to the noun Informer — M-W
noun a person who gives information to the police about secret or criminal activities
"the informer who told the police about that conspiracy has angered a lot of dangerous people"
or informant — M-W
EDIT: For the added example in OP, I'd suggest tattle-tale, telltale, etc. as mentioned in other answers. "a person, especially a child, who reveals secrets or informs on others"
"Sue went to my mother and told on me when I said that I was having an abortion. Sue is such a ____."
Rat — M-W
informal, verb to tell someone in authority (such as the police) about something wrong that someone has done : to betray someone
"The teacher knows what we did, which means that somebody ratted."
From ODO
informal, noun 2.1 An informer
"he became the most famous rat in mob history"
Snitch — ODO
informal, noun An informer
"they thought he was a plant or a snitch"informal, verb 2. (no object) Inform on someone
"she wouldn’t tell who snitched on me"
A person telling on someone may be called a rat, mole, fink, stoolpigeon, tattle-tale, or narc, with each subject to being rendered a verb: ratted, narced, etc.
A lot of people have given you a noun. It seems to me you asked for a verb. In the US, we often say tattle, although it usually applies to children, or childish behavior in adults. It's an aspersion on the tattler. Or, that's how it used to be. I'm old, and the language is quickly outgrowing me.
I notice that one of the answers uses the words rat and snitch. Funny, these two words are both nouns and intransitive verbs, and with help, transitive verbs. That is, a rat will rat, or will rat on you (or even rat you out), and a snitch will snitch, or snitch on you (I've never heard anyone say snitch you out).
The formal word is informing, see NVZ's answer.
Alternatively, there's whistle-blowing, which comes from the noun whistle-blower:
one who reveals something covert or who informs against another <pledges to protect whistle–blowers who fear reprisals — Wall Street Journal>
- whistle–blowing (noun)
Various other informal ones:
telltale Pronunciation: /ˈtɛlteɪl/ ; telltale
…
1 A person, especially a child, who reports others' wrongdoings or reveals their secrets.
See also:
tell·tale (tĕl′tāl′) n. (TFD)
1. One who informs on another; a talebearer.
…
telltale (ˈtɛlˌteɪl) n
1. a person who tells tales about others
(syn.) tattletale, blabbermouth, talebearer, taleteller, tattler, tattletale
gossiper, gossipmonger, newsmonger, rumormonger, rumourmonger, gossip - a person given to gossiping and divulging personal information about others
…
squealer (informal), snitch (informal)
Grass. British informal A police informer. Perhaps related to the 19th-century rhyming slang grasshopper 'copper'
Example Sentences: He then asked who the drug dealer was and when he found out he said, 'I wouldn't do it for him anyway because he's a grass and his supplier's a grass.' — O-D
Grass can also be used as a verb.
Also there is a relatively new term a supergrass.
Supergrass l British informal A police informer who implicates a large number of people: both turned supergrass and were the main prosecution witnesses — O-D
