Quarantine tanks are very very useful and actualy don't cost so much to set up. If you are new to fish-keeping, you probably don't need a particularly large one anyway. Even a small 5 gallon tank is enough and all it needs to have is a heater. Filters are useful but you could always just do daily water changes to keep the water clean. It doesn't need a hood or lights though you may want to cover it with something to avoid suicides...
When it comes down to it though, it is a matter of personal choice. I think it's usualy safer to have one but it's still possible to add fish without problems.
Obviously, the most important part is buying healthy fish. NEVER buy fish from a tank that has other sick fish in it. Also, keep in mind that most LFSs have a filtration system that links ALL the tanks together - if there are sick fish in another tank, leave that shop. One example was when I went to buy some platies the other day but saw that the zebra danios had tb... I bought some corydoras leucomelas instead (because they rarely get tb) and after quarantining them, added them to my tank without problems. If I didn't have a quarantine tank, I wouldn't have bought ANY fish from there.
Another important thing to remember is that fish get stressed by sudden changes to their environment... a quarantine tank enables you to control the water they are in and match it to that of the LFS and then gradualy alter it to the conditions in your tank. If you don't have a quarantine tank, you need to introduce fish to your main tank very slowly.
Usualy, people preffer to float the bagged fish for about 15 minutes to allow the temperature in the bag and in the tank to equalise. Then you need to open the bag (note that if you're introducing a labyrinth fish you should untie the bag immediately and float with the top open so the fish doesn't suffocate...) and start adding a little bit of water from the main tank to your bag. You should continue to do this for about 30 minutes until the fish is in mostly tank water with diluted bag water. With more fragile or sensitive fish, you should spend even more time doing this. Then you need to net the fish out of the bag and place it gently in your tank. You don't want the filthy bag water to get in your clean tank! Let the fish swim out of the net. (note that when moving fish like puffers you must not net them, use a cup - they must be submerged at all times or they will suck up air which can be lethal).
Dispose of the bag and water in the bag and watch the new fish to make sure it isn't harassed. It's usualy best to add 2 fish at a time so that any aggression or unwanted attention is split between the two - this doesn't apply to new aggressive fish.
