Also check http://www.highjumpworld.com/index.htm
It is a confusing site meant for high jump fanatics, but it has lots of links and some might be useful. Webmaster is Joe Faust, who made the 1960 Olympic team as a teenager by becoming the 3rd American to jump 7 feet.
If you find no help, just join a gym and get some guidance from a trainer or coach about lifting with an emphasis on core hip-back-leg strength, stretch a lot, do hurdle-hops, and run stairs and/or up-slopes using long strides and lots of knee lift. Work consistently through summer and fall and you'll see marked improvement, but don't try too much too fast.