When you pronounce the letter "r" in Spanish you have to roll it. For English speakers this can be difficult.
To teach yourself to roll the "r", try saying the word "butter" several times fast. Pay attention to the sound you make and where your tongue is in your mouth. This is about how you roll the "r".
You don't have to roll the "r" much in Spanish words. It does get rolled a little more when it begins a word or when you see a double "r", like in the word "perro".
Try this tongue twister to practice:
El perro de San Roque no tiene rabo porque Ramón Ramirez se lo ha cortado. Listen to it here.
Rolling the "r" may take some practice. But, keep at it, and you will get it!