Getting the guts to ask a long time friend out

I have known this girl for many years now. We are very good friends. I have always had feelings for ever since the first time I saw her, but I never tried anything. Now that we are both older I want to try to take our relationship to the next level. I am continually doubting myself, because of how long we have been friends. I want to ask her out, but I think she wouldn't take it seriously because of our friendship. I don't want to mess this up because if I do I could completely ruin our friendship. How should I approach this?

Tell her you have feelings for her and if she says no, it shouldn't ruin your friendship; if it does, she mustn't be the right kind of girl for you. Because if she likes you, and you ask her out, and she says no, you can still be close friends. Good luck!