This is a tough question. My personal inclination would be to say, don't worry about it. On the other hand, the parable of the widow's mite demonstrates that the Lord is pleased with even the smallest donation from the poor.
If your conscience is bothering you or you feel so moved, my recommendation would be to give a small, token sum (say $5) to the Church or another worthy cause. If, however, you are giving out of fear, rather than joy or at least gratitude, I don't recommend doing it until you can.
With respect to tithing, the Catholic Church does not have a set schedule or percentage for giving. She just says we have an obligation to financially support the Church, in general.
The tithe (a contribution of 10% of income) was part of the law of Moses, which Jesus superseded.
In my personal opinion, it remains a good rule of thumb and starting point, but there is no need to cling slavishly to it under every circumstance.