The can you use will depend on what you want to brew. It you want to brew a bitter the bitter kit you used is fine, what you want to do when adding malts though is balance the malty sweetness they will give you with the bitterness you want.
So if you want to brew a bitter beer and improve it using malts in place of dextrose to balances this out and keep the beer bitter you add some extra hops. This applies to all styles of beers.
However some of us like malty sweet beers and some of us are hop heads and like hoppy bitter beers. If you like malty sweet beers add more malt to the kit and fewer hops, if you are a hop head add more hops and less malt.
Using your brew as an example;
1 can Cooper's Bitter
1kg Cooper's Brewing sugar
500g Dextrose
250g Light dry Malt.
This will be a malty sweet beer; it will have a nice body and mouth feel good head and be low on bitterness. Very drinkable.
If you wanted to increase the bitterness of this beer you could add 15 g of Pride of Ringwood hops boiled in the water with you fermentables for 15 - 30 minutes.
So what tin you use and what you add to it depends on what
you like using malts and hops is a balancing act, to find what
you like experiment and have fun with them. I like malty sweet beers so think your brew will be fine.
Hops can also be used for aroma and flavour as well as bittering if you do a search here you will find lots of info on hops.