Catholics believe that the Catholic Church alone has maintained all the necessary things that Jesus intended his Church to have. However, we also believe that other Christian communities have preserved some or even many of those elements.
We believe that everyone who is baptized with water in the name of the Father, Son, and Holy Spirit is already a Christian, already a member of the Church in some way. They are already in a real, though imperfect, communion with the Church. That is why when a Christian from another ecclesial community decides to become Catholic, we refer to it as entering full communion with the Catholic Church.
For a Christian to become Catholic, all that is required is a profession of faith, followed by completion of their initiation (the sacraments of Confirmation, and First Communion, as needed). Instruction is required so that the profession of faith can be made in an informed manner and the sacraments can be received fruitfully. This instruction may require more or less time depending on the individual's situation.
Contact your local parish for more information.
If you have been Anglican, Episcopalian, or Methodist, there is a particular group within the Catholic Church which has become fully Catholic while preserving elements of their spiritual and liturgical heritage. For more information, see the website of the Personal Ordinariate for the US.