I want to be an actress... how do I tell my parents?

Explain to them what it means to you, and why you like it. Maybe look into finding (depending on how old you are) after school drama clubs or community acting groups in your area that you can get involved with it. They should be able to respect your pion whether they think it is the right choice for you or not. Having a serious conversation with them is really the only way to discuss it. However if they are hard set on you not being one then by the end of your conversation their disposition to your choice of work probably will still be the same so don't get discouraged.