Contact the local Catholic parish in your community. Talk to the priest or the coordinator for the RCIA (Rite of Christian Initiation for Adults). The process depends upon several factors including whether or not you are already baptized, etc.
As for age, if a person is a minor the priest will give you guidance. Those over the age of reason (7) are considered to be adults as far as asking for and receiving baptism, but in the case of a minor whose parents object to them being received into the Church, the priest will likely want to take a pastoral approach and discuss the situation.