Alright internet, let's be brief on the topic.
Effect is used primarily as a noun. "The effects were visible from the back of the concert space."
Affect is used primarily as a verb. "It affects your grades when you stay up all night at a concert."
"It's" is a contraction of "It is." "It's not very uncommon to have difficulty waking up the morning after a concert."
"Its" is the possessive form of "it." "That dog looked upset with me when I tripped over its leg on the way home from the concert. Sorry pooch."
"There" is used to indicate a location. "I passed by the concert space on my way to class the next morning. It was was surprisingly clean there."
"They're" is a contraction for "They are." "They're good about cleaning up after themselves, I guess."
"Their" is used to indicate a possessive. "Their cleaning habits would need to be good, I suspect, to continue being able to rent the space."
"Than" is used in comparisons. "I'd rather attend a good concert than get good grades is something I tried to tell my adviser."
"Then" is used when denoting either a time, or a result. "His response was that I was going to fail, then."
"Imply" is either suggested, or implicated. "I tried to imply that life worth living is worth having lived, not just strictly academic."
"Infer" is used when taking in, or deducing. "He inferred from my explanation that I'd like to fail."
"Principle" is used to describe a core, or primary, tenet. "My principle experience from college was live life while achieving a higher education."
"Principal" is an individual of the highest rank. "However, the principal of the university was called in anyway." (Also, usable as "foremost." "Although, I admitted, my principal reason for attending university was for life experience, not academia.")
"To" is used to denote motion, duration, or limit. "The head of the department told me that I should understand that my grades are going to hell. His glare felt like a punch to my gut."
"Too" is used to indicate an excess of something, or can be used like also. "My adviser mentioned that it's too bad really. Many of my advanced classes would have been fun, too."
"Two" is a positive integer between one and three. "I asked my adviser that if I could bring up at least two of my grades this semester and stay on academic probation. He said that was okay."
"Your" indicates a possessive. "My adviser said, It's your responsibility from this point on."
"You're" is a contraction of "You are." "You're right about that."
"A" is used before words that sound like they begin with consonants. "It was a problem I couldn't ignore any longer. I had to do well in my chemistry class."
"An" is used before words that sound like they begin with vowels. "I wouldn't want to get an F this semester."
Almost without fail, an apostrophe denotes a contraction or a possessive. "It's always harder to come out of bad grades than to keep good grades in the first place. However, it's no one's fault but my own."
The only exception is for pluralizing things with an apostrophe is when pluralizing something that is an abstract concept of itself. Primarily letters and numbers. "It would have been significantly easier to have more A's and B's in the 80's than it is now."
That said, there are a few words that are repeatedly misspelled. There are no tips for these, just memorize them.
Congratulations, definite, apparently, separately, necessary, occurred/occurring, existence, beautiful, and embarrass.