Many think that Paul is talking about giving tithe. What ever a person purposes in his heart let him give it cheerfully. Put that is not what Paul is even talking about. It is true we need to always give in a cheerful attitude. However, Paul is talking about a special offering that he is gathering from the churches as he travels for support of some of the saints who are poor (check out the context of this verse by looking at chapters 8 and 9).
tithing is to go to the local church in order that the needs of that church be met and they have the financial ability to reach out in the community. Tithing has nothing to do with your salvation it is a sign of obedience and maturity. How many churches today are weak and even forced to close their door because saint do not give their tithe. This is not legalism, legalism tells us that we have to do a certain thing to gain God's approval or produce some form of righteousness. Tithing is simply maturity and obedience to God in supporting His work through the local church. Don't get hung up on it just be obedient.

2 Corinthians 9:7