Confusing it’s and its

it’s (with an apostrophe) is a contraction that means "it is" or "it has"

its (without an apostrophe) means "belonging to it"

Confusing who’s and whose

who’s (with an apostrophe) is a contraction that means "who is" or "who has" and, rarely, "who was"

whose (without an apostrophe) means "belonging to 'who'"

Confusing you’re and your

you’re (with an apostrophe) is a contraction that means "you are"

your (without an apostrophe) means "belonging to you"

Confusing they’re, their, and there

they’re (with an apostrophe) is a contraction that means "they are" or "they were"

their means "belonging to them"

there means "that place"

Confusing we’re, were, and where

we’re (with an apostrophe) is a contraction that means "we are"

were (without an apostrophe) is the past tense of "are"

where is talking about a place

Confusing compliment and complement

compliment (with an i ) is to say something nice

complement (with an e) is to match, add to, or complete something

The above is for use as a verb; use as other forms of speech is similar.

Confusing principal and principle

principal (with al ) is usually an adjective meaning "the most important"
(As a noun, it means "the leader," such as the main administrator in a school or head of a company.)

principle (with le) is a noun meaning "a rule or standard"

Confusing criterion and criteria

criterion is singular

criteria is plural

There can be one criterion or there can be many criteria.

Confusing affect and effect

affect (with an a ) is usually a verb meaning "to influence"

effect (with an e) is usually a noun meaning "a result"

If you affect something, you have an effect on it and you have been effective.

There are other uses of the words, such as effect being a verb meaning to create, but if you rework your text to stay with the above, it’s easier.

Using the incorrect:
    "first come, first serve"
rather than the correct:
    "first come, first served"

The version with the "d" is what people usually intend to say. It means someone else is providing you with something because you are the first to get there (that is, you are being served).

Not including the "d" is actually the opposite of what a person probably intends. It means you are the one who will provide something because you are first (that is, you are doing the serving).

Using the incorrect:
    "could care less"
rather than the correct:
    "couldn’t care less"

The version with the "n’t" (or "not") means you're at the bottom of the "care scale" -- you really do not care.

Not including the "n’t" (or "not") means there's more room for you to care -- you might actually care "a little."

Using an apostrophe for a plural, such as:
    "I had many plan’s for today."

An apostrophe usually signifies either possession (such as "John’s" ring or "Jane’s" car) or contractions (such as "it’s" or "you’re" above and personal pronouns, the next item). This mistake would be like writing "hobby’s" rather than "hobbies" or "activitie’s" rather than "activities" -- they are incorrect.

Of course, there are always exceptions, such as acronymns (PC’s) or compound words (to-do’s), because there is no real consensus on those.

The general rule, however, is: do not use apostrophes for plurals.

Using apostrophes in personal pronouns, such as:
    "her’s," "our’s," or "their’s""

Do not use apostrophes for personal pronouns. It would be like writing "hi’s name" rather than "his name." This mistake is similar to the ones above that deal with "its," "whose," and plurals.

Using the word:
    "irregardless""

There really is no such word. What you are in effect saying is: "regardless of regardless of...," which doesn't make much sense.

"Irregardless" is probably a combination of "irrespective" and "regardless." However, both of those mean the same thing, so choose only one of them and use it instead.