It is not a great exaggeration to say that there are no plurals in Japanese. So how does it work in practise?
Much of the time nouns and pronouns are written without any special effort to distinguish singular from plural - you work out which they correspond to by context and what seems natural.
- 猫が好きですか。neko ga suki desu ka. Do you like cats?
In English you wouldn't say "Do you like cat?" so you can assume that the Japanese 猫 neko would correspond to the plural English word cats.
Having said that "there are no plurals in Japanese" there are a number of ways that you can indicate more than one person / thing is involved.
～達 (たち) ~tachi
The standard way of refering to groups of people is with the suffix たち. Technically this is slightly different to what you'd call a plural in English. You can see the difference below.
Looks like plural.
|その人 sono hito||that person|
|その人たち sono hito tachi||those people|
Not quite like plural.
|田中さん tanakasan||Mr Tanaka|
|田中さんたち tanakasantachi||Mr Tanaka and the people associated with him.|
Basically 達 means more like "and company".
～等 (ら) ~ra
The suffix ら works much the same as たち but is less formal. It would sound odd to mix a formal form of address with the informal ら.
俺ら orera : we <- Good (but informal).
わたしら watashira : we <- Bad
わたしたち watashitachi : we <- Good
The suffix たち is primarily used to talk about humans, but just as in English people can talk about their pets and cute animals as if they were people so the Japanese can use たち to talk about them. However, it seems that such kind of speech is childish or illiterate.
ちゃんとしょくじを犬たちにやったかい。chanto shokuji o inutachi ni yatta kai.
Have you fed the dogs (etc.) properly?
(N.B. 犬たち might be "two dogs and a cat" if those are the pets you're keeping. The 'and company' aspect of たち means they don't all have to be the same).
However you can't stick たち on just anything.
ヤだ！ゴキブリたちが部屋中に走れまわってるわよ。yada! gokiburitachi ga heyajuu ni hashiremawatteru wa yo! <-bad
Ick! Cockroaches are running all over the room!
In this case it can be assumed that it is the plural 'cockroaches' so no suffix is needed and たち would just sound weird.
(N.B. わよ is feminine. Men would just use よ. ヤ is short for いや, and (I guess) not many men would use that either).
Japanese has been much influenced by Gairaigo and this can be seen in the occasional use of "s" for plurals, although only in jokey situations. It goes by pronuncation more than spelling so typically ーズ -zu is added to the word.
Here is a classic bit from あずまんが大王 illustrating the use of ーズ as a plural with the word ボンクラ (meaning blockhead / idiot)