Remember that your ultimate goal must be to see your dear wife become a believer in Jesus. It will not help matters if you force the issue against her will. As an agnostic she doesn’t share your faith and love for our Lord. You could “pull rank,” I suppose, and say “We’re going to tithe even though you don’t like it” but that will likely only embitter her. As Christ-follower, you are called to love her as Christ loved the church (see Ephesians 5:21-33).
My advice is to go slowly. Talk it over with her. Explain the source of your faith and why you want to tithe. Pray for God to open the eyes of her heart. Perhaps you can start by slowing increasing your giving over a period of time. If you are only giving a small amount, perhaps it would be better to increase it a little every few months. As your wife sees how God supplies all of your needs, she may become more open to it.
It’s not about money anyway. It’s about your heart–and hers. So I say, take it slow. Talk it over. Give God time to work in her heart. And don’t become embittered against her but show your love to her every day.
See also To Tithe or Not to Tithe.