I know some of you will agree with me on this and others will tell me to take a

but I think it needs to be said. Not only does proper grammar and spelling make a post easier to read, but it also makes your argument more convincing. i.e. why should I believe a person's opinion if they can't even construct a proper sentence?
Here are some basics that come to mind right away
Loose vs. LoseIncorrect:Fighter A will loose to fighter B.
Correct:Fighter A will lose to fighter B.
That chick is loose!
There, their and they'reThere are too many permutations of incorrect usage so I'll only list the correct ways
There (used for placement) - The item is over there
Their - (used for ownership) - That is their dojo
They're - (used in place of "they are") - They're going home
eg. They're going over there, which is their special place
Are vs. ourA consequence of poor pronunciation. I don't even think I need to define each of these.
Your vs. You'reJust like above:
Your - (used for ownership) - Your bike is red
You're - (used as a contraction for "you are") - You're incorrect
Its vs It'sIt's is commonly mistaken for ownership, but really it is a contraction for "it is". Its is the word used for ownership.
IncorrectThat guy is it's owner
Correct:That guy is its owner
It's a red car
Feel free to add some.