Apostrophes to mark omission are used when shortening words. For example, 'I will not - I won't'. When we talk, we often use contractions, so it's also a good idea to learn how to write them properly.
Badly-written English puts readers off. It's also hard to read. Try this for size.
I woul'dn't of tol'd you what i'd done if you had'nt of arsk'd.
What it should be is ~ I wouldn't have told you what I had done if you hadn't have asked. Much easier to read and clearer to understand. Punctuation is useful, but if it's in the wrong place, it may as well not be there at all!
This 11-plus quiz will give you some practice in writing contractions.