I can't recommend this enough. It revolutionised my personal witness and I point as many other people as I can to this.
Listen to Ray Comfort's sermon 'Hell's best kept secret'
I also encourage you to look at the 'Way of the Master' http://www.wayofthemaster.com/ website and look through their witnessing clips and TV programmes. They explain things far better than I ever could. You can even do a course with them if you want.