You can read up on taking the
Home Office Deduction from the IRS website. I suggest reading Publication 587, Business Use of Your Home. Further down on the page is info about Form 8829 (Expenses for Business Use of Your Home).
No, you don't charge/pay rent.

It is a deduction you can take on your tax forms at the end of the year. You can also deduct a percentage of many other items, such as utility bills, etc., but reading Pub 587 and the instructions for Form 8829 will help you understand this better.
This is probably about as clear as mud, but I would have to write a book here to explain it.

So...read the publication for further insight.
HTH a bit,