Can anybody tell me how to pronounce “r ” and “rr” in a correct way？

I am a beginner of Spanish,and  addicted to learning this beautiful  language.Now the problem of pronounciation really bother  me.I can't pronounce the letter of “r",let alone "rr",which make me felt  depressed .Can you tell me how to pronounce them in a correct way? I sincerely appreciate any answers posted here.thx!
I know that practise make perfect,but I have no idea how to practise,pls help me.