Anyone who is baptized Catholic is considered a Catholic. Outside of that, someone would have to be officially received into the Church. The three sacraments of initiation are baptism, confirmation, and communion. If someone has not received those sacraments already from another Church where the sacraments are considered valid, then they would receive them when they are received into the Catholic Church.
As to what must be believed, before receiving any sacraments of initiation into the Catholic Church the candidate declares before the congregation: “I believe and profess all that the holy Catholic Church believes, teaches, and proclaims to be revealed by God.” So, to answer your question directly, yes to enter the Catholic Church one is expected to believe all the dogmas of the Church.
While there are many beliefs that Catholics hold in common with non-Catholics there are also quite a few differences. Catholics believe, but many non-Catholics do not believe: Mary was ever-virgin, Mary’s Immaculate Conception & Assumption, there are 7 sacraments, the Pope and Church are infallible, original sin is not total depravity, Real Presence of Jesus in the Eucharist, and more.
