To offer a trial period for a product go to the Product Price tab and under Payment Type select "Subscription / Recurring Payment."
Additional fields will be opened once you have selected this as shown here:
You will need to enter the number of months between each payment, the recurring product price, then you can set the number of rebills to a limited number such as 12 or you can select unlimited, which will rebill the customer for the maximum time allowed with your payment processor. You will also need to select how a rebill should be handled if a payment fails.
If you want to offer a Trial Period, you will select the number of days for the Trial from the dropdown menu which will open additional fields where you will enter the details of the trial.
Once you have entered your information, you can click the "Save as Draft" button which will save your product in draft form. Or you will need to complete the remaining tabs and then click the "Submit" button on the Review tab.