If you wanted to offer a Monthly Membership Plan which was $0 for 30 days then on the 31st day they were charged the plan price do the following.
Create the Member Plan
Edit Member Plans
The Edit Member Plans page can be found under Members Edit Member Plans
Add a New Plan
The important settings for a trial plan can be found under the 'Member Plan Settings' section.
Recurring Billing: Yes, Recurring Billing
Trial Plan Price: 0.00
Trial Plan Days: Enter a number for the amount a days for the trial period.
Plan Price: Enter the regular fees
Plan Days: Enter a number for the amount a days for each recurring period.
Save the Member Plan
Click the button found at the bottom of the page.