If you want to be more flexible, you need to do static stretching at least 3-5 times a week. NEVER DO STATIC STRETCHES COLD. Warm up intensely first. Do intense parkour for an hour, or run intervals, or something strenuous enough to get the blood really flowing for you. Then systematically stretch out your entire body. If you don't know how to do this at all, then use a routine from a martial arts class, dance class or P.E. or whatever.
If you've never had exposure to any of those... maybe it's time to start. Martial arts, yoga or dance classes are most likely to help you improve flexibility, but make sure you're stretching out of class (when warm) enough to get in 3-5 days a week of stretching.
If you can't afford a class (try your local community college!), then you'll be stuck doing your own research as to how to stretch. Google away, pay up any library fines and start reading! Pavel Tsatsouline's Relax Into Stretch is highly recommended to me and I've been meaning to read it, however I'm not sure if it's good for a beginner.