This should go without saying but it’s surprising how this isn’t always done: the clearest action on a product page should be to buy that product (or add to basket). If you've got the user this far and interested, they shouldn't have to guess at how to give you their money. This page has one primary goal so help the user reach it.
The action should be a button and it should stand out, in a colour that is used sparingly elsewhere on the site. Don't blend it in, don't make the wording unclear, don't prioritise other actions on the page (like add to a wish list). It should be unmissable so it doesn't need to follow the user around the page.
Avoid ever greying out this button, even if you want users to do something like select a size first, as they’re likely to think they can’t buy it. If they click the button without selecting a required size, you should pop up the size selector with an explaining message. When something actually is out of stock, you could replace it with a link to a contact form so they can ask to be informed when it is available again.