Do christians have to tithe?

No, tithing was part of the old covenant. It is never explicitly mentioned in the new covenant. The new covenant requires us to give if required more than the 10% and help those in need. Helping those in need is what we as chistians should do. So if it means we give 10% to our church, great. If it means we need to give more, then we should give more if we are capable of giving more. But there is no law we have in the N.T. that says we have to tithe.
Some people quote that Abraham tithed to Melchizedek. they equate Melchizedek to Christ. So we should also tithe, they say. But the fact of the matter is Melchizedek wasnt Christ. Abraham never bowed down and worshiped him as he did when the three angles came to him later. This shows us that Melchizedek was as human as we are. Also Christ is 'a priest in the order of Melchizedek'. It doesnt says Christ is Melchizedek, which i think it should have if both are the same. 'Without genealogy', only means that Melchizedek cannot be traced to Aaron, the priest. All priests in Israel should be able to trace their lineage to Aaron. Else they couldnt serve as priests. It only shows that there existed a better priesthood in Melchizedek than in Aaron.
In short, give as much as you can to those in need. But dont let yourselves to be deceived that tithing is mandatory in the new covenant.

The origin of tithing