I know how difficult vibrant sounds are. In my case, when I had to learn the French /R/ sound, it cost me blood. But don't worry, Spanish /r/ sound is easier than that.
I think the following exercise can help you, however I'd like to know if you've had some phonetics lessons. I'll try to explain it to you without using technical words:
First of all, you have to place the tip of your tongue right behind your front teeth, but don't press it too hard, otherwise it won't vibrate. Then, let the air do the rest, let it come out.
Tip: Make the sound continuously, trying different "pressure" levels until you get it right. You might not do it well at first, but if you continue practising then you'll become more aware of the control you can have over your articulatory phonetic apparatus and feel more confident. Once you've got the /r/ sound right, try to mix it with the five Spanish vowels. /ra/, /re/, /ri/, /ro/, /ru/
I hope it helps and good luck!