Well, you are free to call yourself whatever you think best describes you!
But if you want to get overly technical ;), anybody who is validly Baptized is, technically speaking, a member of the ‘one, holy, catholic, and apostolic church’ that we profess in the Nicene Creed. As Catholics, we understand that to refer to the Catholic Church. So people who have been Baptized in non-Catholic Christian communities are not in full communion with the Catholic Church, but are still ‘Catholics,’ albeit in an imperfect way.
Now having said that, we usually don’t refer to people who are not in full communion as being Catholics (otherwise we would have to refer to most Baptists, Presbyterians, Methodists, Anglicans, Orthodox, and countless other Christians as ‘Catholics’ – which might be seen by many as offensive or insulting, and, if nothing else, confusing).
Your RCIA teachers will probably talk about some of the terminology with you as the classes get going. An un-Baptized person preparing to receive the Sacraments of Initiation is a ‘catechumen,’ and a Baptized non-Catholic Christian preparing to enter into full communion with the Church (which I assume is your situation) is a ‘candidate.’
When I was in the process of entering into full communion (from the United Methodist Church, in my case), I usually called myself a ‘candidate’ or told people I was in the process of converting to Catholicism. But depending on the circumstances, it was sometimes easier to just say ‘I’m Catholic’ (e.g., when I was in a large group and somebody asked why I wasn’t eating meat on Friday during Lent). It wasn’t really wrong or incorrect to say that, just imprecise. Among Catholics or my closer acquaintances, I was usually more specific.
Whatever you choose to call yourself, God bless you in your faith journey! Coming home to the Catholic Church has been one of the greatest experiences of my life!