Well, I’m glad you are in RCIA and looking to join the Church. God will give you many blessings and you will feel closer to him than ever. The Catholic Church is such a helpful way for us humans to learn about God’s will and to stay on the right path.
Back to your question:
Remember that the Church is against contraception, not birth control. God gave us free will which includes the choice of if and when we want to have children. We just can’t be selfish in our reasoning. At the same time, we have to be open to children so the use of artifical contraception is out of the question.
The basis of this belief is that when God decided to make a woman’s body work the way it does; he put in times of fertility and times of infertility. Unnatural birth control ignores this. NFP however gives us the ability to understand fertility and use that knowledge to determine when to have and when to avoid intercourse, this can work to either avoid, or achieve pregnancy.
Again, as long as we are not selfish in our reasoning this is perfecty fine, and over 99% effective. For instance spacing your children, or having physical, economic, psychological and social issues are valid reasons for avoiding pregnancy. However, not being willing to sacrifice your time, or preferring to go on vacation instead of paying for a child are selfish.