Hi and welcome to the forum
I have to say i agree with the others, a 10litre tank is no place for goldfish, comet goldfish alone can grow to over 15inches long. You should either tank the goldfish back to the petshop or get a much larger tank imediately, since the goldfish will be lucky if they last a couple of months in a tank that small (which is not very good considering goldfish can live to 20-30years old or more).
Strong filtration is essential for goldfish tanks to help keep the water quality good, you should read up on this article in the link below;
http/www.fishforums.net/index.php?showtopic=10099
Goldfish eat plants so any that you put in the tank are unlikely to last long with the goldfish, and even if they survived, they are not likely to thrive without suitable lighting for the tank.
There are also many varieties of goldfish, varying a great deal in size and even individual care requirements.
Petshops are no more obliged to tell you how to be a good fishkeeper than a garage is to tell you how to be a good driver, petshop staff get minimum wage pay and so don't tend to get very indepth training, considering there are thousands of types of fish available in the hobby, you cannot rely on petshop staff to tell you everything you need to know. Not just that, but while there are a lot of well-meaning petshop staff out there, there are also a great deal that have no idea on what they are talking about and will tell all sorts of myths and inaccurate info to sell you any fish or peice of equipment they can etc.
So basically you need to do you own research and research using as many good sources of info as you can and not rely on petshops for info etc
. We here at this forum have no financial incentive for the info we give out, and many of us are more than happy to help you out and answer any questions you have during your fishkeeping hobby experience
.