Apostrophes are certainly tricky, but they do adhere to strict principles. There is no need to guess, or stray, from consistent usage rules.
When letters are removed while contracting two words into one, mark the missing letters with an apostrophe.
- Can not = Can't
- Should have = should've (oh, don't get me started on this one)
- Are not = Aren't
Exceptions include "Ain't," which is arguably not a real word (though I maintain it's short for "Am not"), and "Won't," which is short for "Will not" but the "i" changes to an "o" for some wacky reason.
This rule also applies to names like "D'Angelo," short for "De Angelo," which, when contracted, ought to be pronounced as a single word ("Dangelo"). However, some people choose to pronounce it as though the apostrophe is a pause or accent ("De-Angelo"). This is a hard one to rule on, as names are very much personal choice for the owner.
When a described object "belongs" to someone, then the apostrophe denotes that relationship.
- The dog's bowl. The bowl belongs to the dog.
- My friend's car. The car belongs to my friend.
- John's house. The house belongs to John.
If it's a plural, i.e. there are more than one of something, then an apostrophe is not used.
- Dog bowls. There are many bowls for the singular dog.
- My friend's cars. One friend, who has many cars.
- John's houses. John is a wealthy man and owns several houses.
As an aside, there is the case of "its" versus "it's".
"It's" is a contraction of "it is". The apostrophe denotes the missing letters. But "its" is a possessive, and does not use the apostrophe. This appears to be in contradiction of the rule, but it actually adheres to the rule of "his" and "hers". If you can orient your mind to that way of thinking, you should be able to figure it out and make the mistake less often.
Anyway, back to the rules.
If it's a group of people who possess many things, it is a plural possessive, and an apostrophe must be used in a unique way. After the plural "s".
- The dogs' bowls. Many dogs have a bowl each.
- My friends' cars. Several friends each have their own car.
- John's houses' lawns. The lawns of each house John owns.
And this is the key - you do NOT do this for every word that ends in "s", but ONLY for those that, because it is a plural, ends in the "s".
DO NOT DO THE FOLLOWING:
- The walrus' tusk.
- My dress' colour.
- Jess' house.
Instead do this:
- The walrus's tusk.
- My dress's colour.
- Jess's house.
Three of the same letters in a row may look strange, but that doesn't make it wrong by default. It adheres to the rule, therefore it is right.
Now, there are some official sources, such as the Chicago Manual of Style, that say the former is actually acceptable, but I call shenanigans, and maintain they are wrong.
Unfortunately, because that and similar manuals are used by a lot of modern journalists, we are being overwhelmed by this incorrect usage, and it is fast becoming a widely accepted standard.