Liz explains this very clearly but it might also help if you do some tongue exercises. Your tongue is essentially a large muscle which means that we need to strengthen our tongue to make new sounds. There is a good exercise that will help you to strengthen your tongue and therefore make the 'rr' sound more easily. It is called an "alveolar trill". Here is a link to step by step instructions. http://www.wikihow.com/Roll-Your-%22R%22s I hope this helps :)

'R' sounds a lot like the sound a dog makes when they growl. You know, 'Grrrrrrrrrr', that sound? Make that sound to yourself a few times, and start adding that sound to English words, like 'red', or 'rooster'. Feel how it sounds in your throat, the 'R'. Drop the 'G' sound, then begin to make your 'R' sound shorter. I don't know if this helps, but the 'R' in 'Grrrrr' sounds like the English 'R' pronunciation! :)