Adding sugar before 1st fermentation

Hi all, beginner in homebrewing here...
I'd just like a thing cleared up about adding sugar before 1st fermentation.
I've bought this kit for my first batch and yeah, I know it's crappy and you should avoid kits in general, but I'd like something easy to try for my first time. Anyway the instructions of the kit say you have to add 1 kg of sugar to the fermentation bucket along with the wort but almost all the other sites/resources don't mention anything about extra sugar (apart from the priming just before bottling).
The way I had understood things is that the natural sugars of the grains get converted to alcohol with the help of the yeast, so why should you need the extra sugar? Maybe for quicker fermentation? I'm a bit confused here so any help would be greatly appreciated...
Oh and another thing? What kind of sugar should I use? I've read that table sugar doesn't get converted 100% to alcohol and leaves some residue in the taste of the beer. A friend of mine uses dextrose and swears by it. What do you think?
Thanks in advance...

Those kits only have half of the fermentables needed to make up 6.5 US gallons or 23 litres or 5 Imperial Gallons of beer.
You have to add the other half of the fermentables to make up the full volume.
My advice would be to get yourself some Dried Malt Extract or some liquid malt extract and some brewing sugar.
http://www.makebeer.net/item.asp?idProduct=30&idCategory=110&idSubCategory=0
or this stuff http://www.makebeer.net/item.asp?idProduct=29&idCategory=110&idSubCategory=0
then you would need to add 800g of extra light dried malt extract and 300g of Brewing sugar (glucose) and then ferment the lot.
I would avoid adding pure white sugar as the beer will not be good for that.

Beer kits make alright beer if you do them right, if you add 1kg of sugar it won't be good, if you do it as I've described above you will end up with a pretty decent beer.

Thanks for the quick and helpful answer. I will try your method. Just to clarify, if I did a "natural" brew with grain, hops and all that stuff instead of a kit, I wouldn't have to add any extra sugar because all the necessary sugars would come from the grains, correct?

That's right.

First of all there is nothing 'crappy' about brewing from canned kits. No question when brewed properly they can make a very good beer. Also EionMag provided a good method for making the kits.

Personally for me it depends on the kit and season. For instance in the summer i brew a light ale and add just high malt glucose (liquid sugar) no malt extract. It produces a beer with a thing body and rather dry. I find this very refreshing on a hot summer day.

On the other hand my stout I like full bodied so I add almost all dark malt extract. Experiment and find what combos you like - have fun. Dont discount the quality of beer that can be made from canned kits. Cheers

Thanks for the information guys, you've been a great help!