don't use vinegar to descale, its not strong enough and willl be harder to flush out the taste.
Dezcal is what you would want. Or go to the store and find some citric acid in the canning section but you have to figure out how much to use (probably 1 oz as this is the same amount as the single use Dezcal packets). Dissolve a couple spoons of citric acid into warm/hot water, make sure it is dissolved so it doesn't clog up any small orifices in the machine. Pull water through the group head and fill a cup up to fill the boiler with the descale solution. Turn the machine off. Let it sit for 15 minutes. Repeat until the descale solution has been ran through the machine in stages. Then flush with one or two tanks of clean water. If the water tastes like citric acid then you haven't flushed enough yet.
Also when you are descaling run some solution through the steam wand during the process and also pull out the screen in the grouphead so that any chunks of scale don't get lodged up there and cause a problem.
Also when you are priming the system run a cup of water through the steam wand, this will make sure that the boiler is full. Be careful about trying to run it without making sure the boiler is full, you can overheat the pump and/or burn up the heating element if it isn't submerged. This advice also applies to general operation, after you steam milk, water is removed from the boiler by way of steam. Prime the boiler by running water through the steam arm after use.
Edit- I was thinking about it, thats a low end type machine, if the boiler is aluminum then you might try the cream of tartar that magnus was recommending. No experience with that. I think dezcal is fine in general but I have no experience with the tartar and descaling. A cheap metal used in the boiler could possibly cause some pitting in the metal when descaling. Only way to really know the condition of the boiler internally is to remove it and open it up. This is why I descale using something I know works for sure, like dezcal.
This thread sort of summarized that debate...