Hi Laura, yes, read the FAQ on potty training.
He is too young to mark, I think. At 17 weeks he is not able to be reliably potty trained. If you can not watch him make sure he is in an area that is easy to clean up. Be sure to take him outside frequently (at least every 2 hours) and make sure to praise him if he potties and then play for a few minutes . If he doesn't go take him back inside. This is a time when I like to teach a potty command- Say "go Potty" the second he starts to go every time. Eventually he will associate that command with doing his business. It is great for later when the weather is bad! It takes at least the first year for a puppy to become reasonably reliable.
This is not marking, but your pup is not housebroken and, by the age of 17 weeks he should be. That means you need to revisit your housebreaking strategy, because something you've done is not working. A bit of detective work on your part is needed to sort things out.
Hopefully the FAQs and Bev's suggestions will help.
I agree, he's too young to be marking. Go back to the basics with housetraining.