I'll admit here that I'm no grammar, punctuation, or spelling teacher. Here is a list of words that, in my own opinion, are confused and misused most often when writing. And yes, I am guilty of a few of these myself.
Than and Then: Than is used when two things are being compared to each other. Ex. I can run faster than you can.
Then is used when telling something is happening or has happened. Ex. I brushed my teeth, then crawled into bed and drifted to sleep.
Alright and All Right: As shocking as it may seem, I have been told and read that alright is not a word. It's always all right.
Alot and A Lot: Same deal here, alot is not a word. It's always a lot.
In and Into: In means where something is right now. Ex. I am in the basement.
Into implies movement. Ex. The dog bounded into its cage.
Its and It's: Its is possessive. Ex. The owl flapped its wings and darted through the night sky.
It's is a contraction and means it is. It's time to come inside now.
Loose and Lose: Loose means wiggly. Ex. The screw is loose.
Lose means not able to find something. Ex. I put my homework in my notebook because I don't want to lose it.
These are just a few to watch out for, beware though, there are many others.