@SuMeBe—
On the Internet, there are a lot of people who don't know how to use an apostrophe appropriately, so I'll explain it here.
The apostrophe has two uses, one evolved from the other.
The first use is to indicate the omission of a letter or letters,
where the omission occurs. For example, the contraction “don't” indicates the omission of an ‘o’ in “do not”, and one should
not write “do'nt”. The word “
'tis” omits the first ‘i’ in “it is”; that's why there is an apostrophe at the start. In the case of “li'l”, one indicates the omission of “tt” in “little”; the omission of ‘e’ is ignored because it was silent in the uncontracted form. One should
not write this as “lil
'” or as “
'lil” (unless one is referring to the nickname of an ignorant rapper). In old-fashioned spellings in which a letter was sometimes pronounced and sometimes silent, it might be spelled with an apostrophe in the second case. When “damned” started to be pronounced /dæmd/ instead of /ˈdæməd/, it often was written “damn'd”.
The second use is in genitives. Originally, many of these were actually written with “es” instead of with “'s”, and the apostrophe indicated that the ‘e’ was being omitted. Then, even when there hadn't been an ‘e’, the genitive was formed with “
's” just to be more regular. And, when a genitive was formed from a word already ending in ‘s’, and an extra /əs/ sound wasn't added for the genitive, sometimes an apostrophe by itself was used just to make it clear that a genitive was used.
While I'm at it, let me noted that
none of the pronouns use an apostrophe in the genitive form. That's how you can remember not to write “it's” for the genitive of “it”.