There are many basic stretch exercises on the internet.( example ; https://www.youtube.com/watch?v=HVyNSsnp... )
Many martial artists practice yoga for gaining flexibility too , it's more fun too practice in group.
But NEVER over do it ,that means while you stretch you need to feel it burn but not hurting.
I started training my flexibility only about a year ago when i was 19 , never did any martial arts before and was as stiff as a board. Today i notice i'm much more flexible.
Another extremely efficient training method is using a leg stretcher like this http://www.martialartsmart.com/20-60.htm... expensive but worth it.
or if you have a less bigger budget these work well to http://www.martialartsmart.com/20-76.htm...
Good luck
I need to be flexible as I'm stating TAEKWONDO but I don't think I'll be able to do the high kicks. I know it would take months but I'm not sure even then I could. I can't lift my leg much and straighten it. What can I do. I tried normal stretching for months before I started and it barely made a difference. If it makes any difference I'm a 13 year old female.