The practice of tithing was under the Law of Moses. Christians today don't have to fearfully practice it because we don't have to do anything to be accepted by God after we accept his Son Jesus as our Savior. The tithing principle, however, is a good guideline for those who need structure in giving.
While tithing required 10%, the scriptures teach that it is good to give more than that when we feel free to do so. However freely we give it will be freely given back to us (Matthew 7:2). This is a statement on how much we can expect back, not on how much we need to give.
Today, it's not about tithing. It's about giving abundantly to others out of our own feeling of abundance. It's giving in freedom. The type and amount of that free-spirited giving varies from Christian to Christian. Our Father wants us to give from the heart, in the amounts and the frequency that we feel comfortable to do. Give how, when, in what way, in what amount and to where you are inspired to give.
If anyone attempts to make us feel ashamed because of our monetary giving or the lack of, their spirit is not of God. If you are not giving today but would like to, don't despair. God sees your heart and he will surely help you to get to your goal. He works with us, not against us. He does not push us, he understands us. Giving is a choice in freedom. Be comfortable and at rest in your freedom in Christ Jesus.