On this site you will be introduced to text mining with R. After 45 minutes you will be able to venture out in the text mining world on your own! 
The introduction falls in four parts seen here under. Each part contain a video and a text section. I recommend that you watch the videos as they are more thorough. The text sections should be viewed as notes on the steps presented in the part and the code from it. Besides that there will be links for additional materials here as well.

Good luck!