When designing a landing page to promote a product or service online, you're ultimately pointing users toward one goal. That goal most often relates to generating business via sales or leads. You may want users to purchase a product immediately, or you may simply want them to sign up for a mailing list. Whatever the goal, you want to ensure that every piece of the user experience works toward fulfilling that goal.
If you don't yet have goals in mind, start by defining goals. Are you seeking to generate a 10% increase in qualified leads? Are you looking to build sales by 20%? Establishing clear key performance indicators based on what will benefit your business will ultimately help you understand how to properly approach a landing page.