You need to do some dynamic stretching. Just swing your leg up to a comfortable height, within your current range of motion. Do this 8-15 times, gradually increasing the height until you've reached your maximum extension, as determined by your curent level of flexibility. Be careful not to throw your leg up so it reaches its limit at momentum; you'll tear a muscle that way. Control the kick.
Do this to the front, back and side with both legs. Do it every morning upon waking, and at least once again during the day. The more often you do it, the quicker you'll progress
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