The Catholic Faith is the one, Holy, Apostolic Church founded by Jesus Christ. People tragically fall away for a number of reasons, but there is a way to return to the Sacraments!
Catholics Come Home is a wonderful apostolate for people who have been away for a while. Check out the webpage for many great resources.
If you are in a marriage that is not blessed by the Catholic Church, please contact your priest to work to remedy this situation. If there is no previous marriage, usually this can be a simple remedy called a convalidation. Until your situation is resolved, you must abstain from receiving the Eucharist, since for Catholics, the Sacrament of Marriage must be done under certain conditions (i.e a marriage performed in another denomination or by a judge is not a valid marriage).
Here is a helpful website that discusses aspects of marriage, including ideas to improve your marriage.
If you are not in such a situation, go to confession and let the priest know about how long you have been away. If it has been a number of years, it may be helpful to contact a priest to arrange an appointment since this confession may take a little bit longer than a person who is regularly going to confession.
Before you go to confession, check out:
Examination of Conscience
Forgiven: The power of Confession