yoga is most definatly the best way to increase flexibility. there are other ways to strech, however yoga will improve health, digestion, consentration, relieve stress, strengthen muuscles, improve balence, burn fat and its a lot more fun than basic sports streches.
If you can, get a cheap yoga mat, and a DVD. Make sure you get a beginners dvd cos its harder than it looks.
The best thing would be to go to classes, even if you dont like classes I strongly advise it once or twice before you train on your own.
If your not in good shape, you may wanna consult your GP 1st.