Hi! To pronounce the letter "r", just put the tip of your tongue in the middle of the palate. At the same time, exhale air -by your mouth- a little strong. So your tongue will vibrate and that -the vibration- will produce that characteristic sound. 

There's a whole article about this: https://www.italki.com/article/865/R-Con-R-Cigarro:-The-Letter-R-is-Coming-Up

Pretend the letter is a "d" and try using that sound instead!