During the time of Cain and Abel, giving first fruit to God can be seen through their offerings. They offer their best harvest and their best sheep to Yahweh but only the one who gave his best to the Lord made Him happy (Genesis 4:3-5). Today, we don’t need to do such things since not all of us are farmers but we can apply the principle of giving first fruits by following this principle- first things first should always be given to Him- it means giving to Him our best in everything.
Today, most people give their best in the form of tithes. If  you are too busy working, then make “giving” as your ministry. Give your best tithes to help your Church spread the word of God and to support its workers. If you have no work but have plenty of time, offer your strength to Him by doing things that can help the church or the community. Just give your best to the Lord and do not worry about things that is not within your control. Don’t worry of getting hungry, because a good giver will bear fruits more than he or she will ever need.
I remember a missionary friend I met few years ago. He is a Korean missionary here in the Philippines. He said his family is poor yet he decided to continue giving his life to the Lord because it is the only way that he will bear fruits. He doesn’t worry about his parents in Korea because he knew God will provide for them . All he is worried about is becoming fruitless man of God.

Remember Proverbs 3:9-10, “Honor the Lord with your wealth and with the first fruits of all your produce; then your barns will be filled with grain, and your vats will overflow with wine”. So give what is due to the Lord and he will supply your needs. Trust in Him.