In my opinion, the best thing to start out with is some Jamey Abbersold books. How it works is that you have a written melody with the chord changes above each measure so that you can see the chords being played. Also, right underneath the melody you have to scales written out right there for you to see so that beginners can actually see the notes that you can and should play for each one of the chord changes. The book includes a CD that has a backtrack for each song. You have a complete rhythm section to play along with: drums, bass, and piano. These books are very affordable and are definetly my favorite way to practice improvising, wether you are a beginner or a pro.
The website is easy to remember:
www.jazzbooks.com
p.s. I would recommend you start out with book #2: Nothin' But Blues. It's perfect if you're just starting out.