The Spanish letter R can be difficult for students. It is pronounced by rolling or trilling the tip of the tongue against the roof of the mouth:
- Open your mouth about as wide as you would to say "oh," but don't purse your lips.
- Position your tongue so that it lies straight, touching neither the top nor the bottom of your mouth.
- Bend the front half of your tongue and place the tip slightly behind where you would put it to pronounce T or N. The tip of your tongue should be lightly touching the back of what linguists call the "alveolar ridge" – the flat "plane" between your teeth and the "canyon" where your tongue usually sits.
- Tense your tongue, but only let it touch this spot lightly.
- Exhale through your mouth, allowing your tongue to vibrate against the your mouth.
It is essential for your tongue to be tensed, but for it to touch your mouth only lightly. If you are touching this spot too hard, you'll end up saying something like "D, D." Too loose, and you'll just whistle.
When R is at the beginning of a word, it is pronounced like RR.
Please note that this explanation is only a guideline for the Spanish that I know, which is Castilian Spanish. There are many regional variations in Spanish pronunciation.