What does "devotional service" mean? How can i perform it?
Devotional service means activities performed for the pleasure of Lord Krishna, with devotion, and ultimately not mixed with any desire for personal pleasure.
The best way to begin devotional service is to try to offer everything you do, or some portion of everything you do, to Krishna. For example, if you cook, offer the food to Lord Krishna before eating. If you work, then take some of the results of your work and offer it at the temple for the seva of Lord Krishna! If you have some skills, offer to help at a temple and use your skills for the benefit of the Deity, the temple, and the devotees.
In the 12th chapter of Bhagavad-gita, Lord Krishna gives several different alternatives as to how to begin the process of devotional service. And He also says that even offering a little water, a fruit, or a flower with love to Him constitutes devotional service. So does prayer; specifically, chanting:
Hare Krishna Hare Krishna Krishna Krishna Hare Hare
Hare Rama Hare Rama Rama Rama Hare Hare
. . .is devotional service, and will clean personal desire from the heart and allow you to focus on just rendering loving service to Krishna.
The ultimate barometer for devotional service is the love with which the offering is made. Lord Krishna has everything, and so many people serving Him, but anyone who renders service with love, free from envy, becomes His friend and devotee.
I hope this is helpful.