Usually the first of the postures to be taught by a Yoga Guru, Padmasana is one of the basic Asanas but needs a lot of focus to execute. For every Asana, it is important to be relaxed. Other complex Asanas are taught once you are comfortable with this one. Thus, it is considered as a significant part of the building block of your journey towards excellence in Yoga.
Though a basic posture, it is the most important one. It is most suitable for Dhyana(Meditation) as the posture helps to be stable for a long time and helps us to relax as it applies pressure to the lower spine. This posture even improves digestion which is a bonus. And it obviously reduces blood pressure as it helps in relaxing.
As a beginner it is observed that a person feels pain on the shin, thighs and other muscles trying to be stable in this posture. You can use some kind of elevation beneath your pelvis with help of a pillow so as to reduce the pressure and gradually eliminate the elevation. This will be less painful and will help you in getting the posture right.
This post will not teach you how to do the asana though. Really sorry for that. I am a firm believer that you need a teacher to learn something like this and internet is not in such a position. This post is just emphatic on how it is beneficial and some tips on how to do it in an easier and less painful way.