The concept of tithing is based on the assumption that there is income. Anyone who is alive has income of some kind. Are you getting money or material provision in any form such as unemployment, Social Security, welfare, help from a church, family, or friend, etc. If the answer is yes to these or anything else, then that is your income.
You can always tithe, even if you have no cash. Where are you getting your food or clothing? Give 1/10th of it back to the Lord by giving it to other people who need food or clothing.
Often people who live in abject poverty (as the Macedonians did in II Corinthians 8) are the greatest examples of giving. And the poor widow had only her two coins but gave them both away.