It sounds as if you have just recently made that decision. My advice is to hold onto the call and wait a bit. If it is true, it will grow, if it is not (and that's ok) then you will be steered in the direction you are called as long as you are putting God first in your life.
Let it grow in your heart and I would say keep it a secret some more until you feel ready to speak with your parents about it. The reason why I state that is because you asked the question of how to tell your parents... it seems because you asked, you may not be ready to do so, and that's ok. They don't need to know yet. Let your love for God grow some more. Keep growing in the Lord Jesus and He will provide the proper window to speak with your parents. (And that may be the reason you feel apprehention about telling your parents- it may not be time yet.. I'm presuming from what you have wrote.)
Please take that time... I wish I did because I ruined everything I had when I told my mother, (my father is not catholic) who did not look upon the idea as something noble and did not keep it a secret. My family then started to act 'funny' around me and making statements about how most religious (priests, nuns, friars, etc) are really gay or mentally ill... these topics they never talked about until my mom leaked my confidence in her. ( We are supposed to be a Catholic family.... and I gave them the benefit of the doubt that they had faith. I found out we are not. Doubts ensued... and I wish I would have waited. They have no clue on how to look upon relgious life except for what media tells them, so there is no understanding in them for this.
Pray for the gift of understanding in your parents about religious life. Also, because your parents know you better than any one else, pray that you will hear and take note of any of their objections,( so as long as they fit with Catholic teaching, that is.)
If you suspect your family is like mine.. my advice is also to get ADVOCATES and support first. (and not on mesage boards either.) Look for people who are going to tell you the truth, even if it hurts. If you know of any people who have shown to you that they are strong in their faith and that they put God first, talk with them. But mainly, you should first try to make sure that this calling is TRULY from the Lord. That takes time.. I don't thiink the Lord would want anyone rushing into religious life as much as He would want someone to marry someone on a whim. He would want you to be sure, just as any man who really loves you would want for you also, with a full heart of devotion and readiness.
So, please if my advice helps... keep it within you until you feel safe and secure to make the next move. Let your love for God continue to grow. Let your interest grow, Explore by reading books on religious life (pious ones, that is) and possibly take some visits to monasteries of interest. I had read something about one saint's vocation to religous life and his advice for him was to keep it a secret because too many people think its stupid and meaningless and will rip you and your vocation to shreads. Be cautios.
Hope that helps some. Best Wishes and Congrats!
May the love and guidance of the Lord Jesus Christ be with you!