Here are common writing mistakes to avoid:
#1 Their, There, They're
Their is a plural possessive. There can be used as a pronoun or in reference to a place. They're is a contraction (they are).
The students have their books.
There is an old house.
The kids want to go there.
They're going to school today.
#2 It's, Its
It's is a contraction for it is or it has. Its is a possessive pronoun.
It's my birthday.
It's been a warm day.
A cat is possessive of its territory.
#3 You're, Your
You're is the contraction for you are. Your is a possessive pronoun.
You're my best friend.
Your article is fascinating.
#4 Affect, Effect
Affect is a verb ("to influence") and effect is a noun ("result").
The heat will affect my garden.
The medication has a negative side effect.