Here are words that everyone should know the difference between:

There/their/they're
To/too/two
Your/you're
Its/it's
Here/hear
Herd/heard
No/know
Write/right
(If I missed a few, please tell me)
Now here are examples of each:
-Look over THERE! (Place)
-THEIR puppy is cute. (Ownership)
-THEY'RE doing something right now. (Contraction, they are)
-Johnny was going TO the dance. (Preposition)
-Clarissa wanted to go TOO. (In addition to a previous statement)
-She had TWO dresses. (A number amount)
-YOUR books fell out of YOUR locker. (Ownership)
-YOU'RE being ridiculous, Noah. (Contraction, you are)
-ITS face glowed in the moonlight. (Unspecified he/she, possessive)
-IT'S too cold outside. (Contraction, it is)
-Look over HERE! (Specified place)
-I HEAR you laughing! (Reference to sound)
-There is a HERD of sheep nearby. (A group of animals)
-I HEARD you whisper. (Past tense of "hear")
-I say NO, Bob. (Answer to a question)
-I KNOW that, Bob. (Previous knowledge about a subject)
-I started to WRITE a book. (Action)
-That answer is RIGHT. (Correct, or can be used as a direction, like "Turn right!")

All of these words are "homonyms", words that sound the same but are spelled differently.

Another lesson in grammar is when to use either a/an. There is a simple way to tell. Use "an" if the word after it begins with a vowel. Use "a" if the word after it starts with a consonant.
-The child picked up AN orange.
-The child picked up A banana.