Study the autobiography of Mahatma Gandhi. He never made any difference between menial service and dignified work. Scavenging or cleaning the latrines were the highest yoga for him, the highest puja. He annihilated his illusory little 'I' through this service.
Many highly educated persons joined his ashram to learn yoga under him. They thought that Gandhiji would teach them yoga in some mysterious manner, in a private room. They thought that he would teach them pranayama, meditation, awakening of the kundalini, etc. They were asked to clean the latrines first.
Gandhiji used to mend his own shoes. He himself used to grind the flour. He would take upon himself the work of others when they were unable to do their allotted portion of work. When an educated person, a new ashramite, felt shy to do grinding work, Gandhiji himself would do the work in front of him. And then the man would willingly do the work the next day.
Try to do daily as many virtuous actions as possible. When you go to sleep have a review of your day's actions. Mark them in your spiritual diary. Performance of virtuous actions is the beginning of spiritual life.
Repeat mentally or silently with the breath, the Lord's name such as Hari Om, Sri Ram, or your own ishta mantra. Do this even when you are working in the office: A strong habit of repetition of the mantra will soon be formed.
Karma, according to Jaimini Rishi, is the performance of agnihotra and other vedic rituals. According to the Gita any action done with niskamya bhava (unselfishly, motivelessly), is karma yoga. Lord Krishna says: "Work incessantly. Your duty is to work but not to expect the fruits thereof." The central teaching of the Gita is nonattachment to work. Breathing, eating, seeing, hearing, thinking, etc., are all karmas.
Service of your guru or a mahatma (saint) is the highest form of karma yoga. This purifies your heart quickly. Contact with a great soul or guru has immense advantages. You will benefit by the magnetic aura of the great personage. You will draw inspiration from him.