First off, let me say that I do not believe that I am the person to consult when you have a grammar problem. I don't even like reading grammar posts; they tend to read holier than thou. Yet, I had thought that by now that most writers knew that they had to edit their own writing-correcting their own grammar, making things clear and concise. Unfortunately, lately I keep reading things that are clearly unedited.
Here are a couple of the mistakes that I keep seeing over and over again, even with accomplished writers:
There/They're and Their
If it has to do with place, or location, if it can be replaced with the word here add a T and make there. Example: Put it here. Put it there.
If it can be replaced with the phrase, they are, take out the a and replace it with an apostrophe, and close the gap between the words to make one word-they're.
Example: They are too good to be true. They're too good to be true.
If it has to do with ownership, think of the word, heir. Add a T for their.
One of my English teachers long ago put this one into perspective for me. She said that we don't expect to add an apostrophe to the possessive of any of the other first, second or third persons so why would it be different? Think about this--his, hers, yours, mine, ours; so in keeping with all of that, you have its as the possessive. No apostrophe. (Yes, that is a sentence fragment. I believe in breaking the rules for emphasis.) An example might be: the dog has its own bed.
Whereas, if you can replace the word in your mind with the phrase, it is, then you need the apostrophe. Example: It is going to be a good party. It's going to be a good party.