Oh dear, please don't go back to that place, the advice you were given is so wrong.
Have a read through this especially the section about fishless cycling. You need to get a source of ammonia, you can get this at homebase or Nisa stores. You basically need to build up enough good bacteria to support your fish, otherwise they will most likely suffer often fatal ammonia and nitrite poisoning. The link will explain.
Test kits are an absolute MUST. Get it on Amazon for half the price. You should never have Ammonia or Nitrite over 0.25, hence you need to test, especially in the beginning because you need to react if it goes over, as in doing huge water changes.
If you know any fishkeeper close to you, you could ask for mature media. This is the foam in the filter. If you know someone, they can safely donate you up to 1/3 of theirs to put into your filter and you are basically cycled. If you are close to me (I'm in Staffordshire), I'm happy to donate some to you if you like. Some independent fish shops are also happy to give you some of their media, so you have a massive kick start.
One goldfish needs at least 140l, depends on what kind. Common ones need 180l for one single fish, so please don't put them in a bowl. If you want to keep that small tank, please consider getting a siamese fighter fish (betta). One male will be the perfect tank inhabitant and they are beautiful with so much personality. Mine always comes to the glass to say hello to me.
Real plants can be washed under tap water. Snail eggs are in a jelly substance, so just make sure there are none. If you ever have snails, get some assassin snails and they will eat them and assassins don't breed as much as pest snails.
It might all sound daunting now, but don't be put off. Fishkeeping is so rewarding. I started off like you and had no clue, put all the fish into a small tank as P@H advised, they all died and then I realized what was going on. Read up on cycling, learnt and made it all better. I started with one tiny tank more or less exactly a year ago. I know have eight! So beware, it's addictive 