It really depends on what you're looking to offer, but generally you should allow people to try before they buy, encourage and help them while they try it out and then ask for the business.

A common tactic is to first collect a form of payment, and then the users can start their trial period. This works well for both customers and businesses. Customers get to test drive the product and the majority forget to cancel before the trial ends, leading to paying customers. A 14-day trial is typically sufficient.