Dont dump them in a lake. They grow giant and eat all your other fish

Be sure to change out filter when recommended, be sure they are fed them daily (otherwise they'll eat each other), always get new pebbles when cleaning out the tank, and be careful when moving the fish while cleaning (if you must move them out). If you put them in a different water environment without "temping" it or making sure the ph level is similar, you will in a sense shock them. These are typically the problems I found to be a hassle.

And just a reference:
http://www.wikihow.com/Take-Care-of-Goldfish

Goldfish from WalMart are sickly and live for like a day.
Ones from Pet stores a little more expensive, but they do live longer.
The ones that have always lived the longest for me are the ones I have won from a carnival game. (weird right?)

I would recommend getting a snail to put into your tank too to help keep the glass clean. Either that or a sucker fish which will also help keep things clean.
These fish are swimming in their own toilet, so keeping the place clean is a chore.

What size tank are you looking at getting?

Probably somewhere between 15-30 litres

Goldfish are hardy fish, and I love them :) have a nice big tank with some shubunkins and comets in it. They do well in a roughly 7.0 to 7.5 ph, but a bit below of over isn't much to worry about. They grow pretty big, so make sure you either have a large enough tank, or will be getting one later on. for a few goldfish, I'd say they should have a final home of at least 35-40 gallons(not sure what that is in litres. Partially they also need the space since they also produce a lot of waste, and more water to swim in means the water dirties slower and requires less frequent cleanings to keep them healthy. I've never in my life witnessed goldfish cannibalizing each other from skipping a meal or two, so I can't vouch for that comment. Just feed a little for each one once a day, maybe a small snack later in the day, too. They can eat flakes, pellets and I even offer blood worms. Temperture-wise, they are easy to please, they do well with just room temp water, though is your house is really hot in the summer, it way be a bit much for them, I believe about 75F is about their natural good point temp-wise.

One said to change to pebbles each cleaning....maybe I read that wrong, but if they meant to buy new pebbles each time, that is not true, the pebbles are good for like...ever, though they will lose some of their coating over time. Keeping fish in too small a tank can lead to poor heath and possible deformities from the cramped space, so please care for them. I find it's good to offer some mushed up thawed peas (not fresh, must be precooked) and throw it in the tank. It's supposed to help with digestion and any gas caught in their swim bladder, which can cause uncontrollable floating. They love them, too. People often overfeed, but they don't require a lot of food, they are just little piggies and will eat till they burst, but too much food is extra waste and dirties the water quicker. I also don't recommend live plants in with goldfish, they are grazers and will eat the plants bare, so fake ones work better.

I have had goldfish for most of my life, so anything I may have missed feel free to ask. Good luck with your new little fishies. I find goldfish very pretty, active and often with personality if you observe them enough.