Where can I start speaking a language if I can already understand it

Kind of strange if you read the title, but just listen. I'm a Korean teenager and I want to learn Korean. My parents are semi-fluent in english, but because Korean is their first language they usually speak to eachother using it and sometimes to me. So I have a decent understanding of Korean. I can roughly translate most sentences and such, but because I never spoke it, I can't form sentences and can barely remember words that I don't use very often. Most people just assume speaking comes with understanding, but for me its like they're two completely different things. What do I do and where do I start?