Like everyone else said, use the language, and you'll find yourself learning it naturally. Find the smaller, slower channels and get a feel for it and practice. Just never be afraid to try. The fleets I run have a LOT of people who were simply afraid to talk or type because of their language issues.
We told them straight up that, despite being native English speakers, we still spoke it horribly (as a form of encouragement to him at our expense). We're always accommodating of them and help them learn the language if they really want to. I think you'll find the same is the case for the RP community. We have one person here who knew very very little English, but they've just kept plowing along and growing their English skills fine and dandy. I'm sure you'll be the same.
tl;dr don't be afraid to test your English and learn just because you aren't good at it. Practice makes perfect, and I'm sure most here would like to help you practice.