go to your local garden centre and buy 1 or 2 thin garden canes ( the type used to make plants grow up straight). Tie them together so that you have a thin pole about 4 foot long.
Ever done high jump at skewl? Right same principle, place the pole across a couple of 'anything' so that its a foot off the ground. Jump it, if you fail you will just flick the pole to the ground. Once you are confident , raise the bar. Eventually you should be able to jump the bar once it is laid between a couple of wheelie bins ( if you bend/raise your legs up). Then move on to more solid objects, onto or over walls.
As for actual jumping technique. Have a look at urbantrickers tutorial on front flips. The 1st stage of which gives very good tips on jumping high and I dont think many could better that.