Research past portfolios, accepted and rejected. Look for petterns so you have an idea what Sheridan is looking for. This 2016 one is a must-see. It scored 100%.
Also, make sure you apply at Ontario Colleges first. From there, Sherdian will know you applied and will send you portfolio requirements and such. Send transcripts there as well.
And one more thing, this zine by Illumous (Class of 2021) is really really really good.
Otherwise, here's a list of resources that helped me:
What? You want books? Okay...for the old school peeps: