Baptism: Gateway to the Christian Life
May a Catholic receive Holy Communion if the only sacrament they have received is baptism?
Baptism is “the gateway to life in the Spirit (vitae spiritualis ianua), and the door which gives access to the other sacraments" (the Catechism 1213). So, if a Catholic has received only baptism, he could also receive the Eucharist, given adequate preparation. If he is an adult, he might also receive confirmation on the same occasion, given that baptism, Eucharist, and confirmation are the sacraments of Christian initiation (the Catechism 1212).
A non-Catholic Christian would have to go through a more formal catechetical process, typically the Rite of Christian Initiation of Adults (RCIA), to enter into full communion with the Church and thus receive the Eucharist (see the Catechism 1232-33).