A ‣ Monosyllabic informal words
Informal versions of words are often short and monosyllabic (consist of one syllable).
The cops have arrived. [police]
The house has two loos. [toilets]
It cost me ten quid. [pounds]
I'll help you peel the spuds. [potatoes]
My bike's been stolen. [bicycle]
I always go by tube. [word used for the London Underground]
His dad spent ten years in the nick. [prison]
I'll get a cab from the station. [taxi]
B ‣ Clippings 
Shortening a word tends to make it less formal, though these forms are acceptable in a wide range of contexts.
I'll meet you in the lab(oratory).
She works for a tech(nology) company.
What's on telly tonight? [television]
Have you seen that mag(azine)?
We should put an ad(vertisement) / an advert(isement) in the (news)paper.
Can I use your mobile (phone)? I need to (tele)phone my brother.
There's milk in the fridge. [refrigerator]
Her sister's a vet(erinary surgeon).
Would you like to be a celeb(rity)? [famous person]
C ‣ Slang 
Slang is extremely informal language. Slang helps to make speech vivid, colourful and interesting but it can easily be used inappropriately. Slang is mainly used in speech but it is also often found in the popular press and in literature. Slang changes very quickly, so some words go out of fashion, but you may meet them in novels and films. Some slang expressions may cause offence to some people. Here are some examples you may hear or read.
Expressions for money: dosh readies moolah
Expressions for food and drinks: nosh grub sarnie [sandwich] cuppa [cup of tea]
People obsessed with computers or other equipment: nerd geek anorak
Jobs: quack (doctor) shrink [psychiatrist] the old bill [the police] squaddie [soldier of a low rank]
Language help
If you are interested in slang, you can find more examples in films or in the tabloid press but it is probably safest to keep it in your passive rather than your active vocabulary.