Yajna is often translated as sacrifice, though a better sense is offering: giving something of value, time, effort or attachment, for a purpose beyond oneself. The Gita broadens the idea from ritual fire ceremonies to the whole of life. Work, giving, study and self-discipline all become yajna when done in a spirit of offering and not of grabbing.
Gita 3.9 says action performed as an offering, free from selfish attachment, does not bind, whereas self-centered action creates bondage. Gita 3.11 describes life flourishing through reciprocal exchange, and Gita 3.12 says that enjoying gifts without gratitude or giving back is a form of theft. Gita 4.31 calls a spirit of sacrifice essential for worldly harmony and spiritual advancement, and Gita 9.16 says that every element of sacred action, the offering and the fire, is permeated by the Divine.
The Gita also lists many forms and a measure of purity. Gita 4.25 describes how different seekers approach growth through different forms of sacrifice, so respect diverse paths while remaining sincere in your own. Gita 17.11 says a sacrifice offered as duty, without desire for reward, belongs to the quality of goodness, and Gita 18.5 says sacrifice, charity and austerity are purifiers and should be kept up, not dropped in the name of renunciation. Keep daily practices of giving, service and self-discipline.