Call or email your MPP
Your Member of Provincial Parliament has both a legislative (Toronto) office and a constituency (local) office. They can be reached at either office. If you are writing about something personal, write to your MPP's constituency office. Send letters about a law, or something to do with the government, to the Toronto office at Queen's Park.
Find your MPP online here.
Scroll down to the bottom of the page, type your postal code into the search bar, and your MPP's contact page will come up.
You can write to your MPP at either the legislative or constituency office. See above for finding contact information for your MPP.
You do have to put postage on a letter to your MPP or the Premier.