Hi Peter, thanks for posting, and welcome aboard

The '1 tonne challenge' is best executed by using a weighted barbell, which you can lift in several different ways. You can use a rock, or log, or anything that tickles your fancy, but for the sake of safety and efficacy, I recommend a weighted barbell.
The movement I recommend using is the 'Clean and Jerk'. It is functional, safe, efficient and very effective. You bend at the knees and hips and keep your back in a relatively neutral position. Do not keep your legs straight - that can be quite dangerous, and it isn't very functional either.
The Clean and Jerk is an advanced exercise, and must be done correctly, in order to prevent injury and to maximise the efficacy of the exercise. Go here for an excellent demonstration video (scroll down to the Clean and Jerk videos);
http://www.crossfit.com/cf-info/excercise.htmlAlso, there is no need to lower the weight under control, you can drop the weight after you have reached full extension - just like they do in Olympic Weightlifting.
And remember... be safe, use a light weight at first (don't get greedy) and most importantly, have fun
