John had some very good advice for supermarket orchids. I still get tempted by them and bought another just a few months ago.
I agree Matt above, as a beginner it's a good idea to try on orchids that are not very expensive (although online ones from a good grower are often comparable price).
I bought supermarket ones for a long time, then for arround 6 months I kept saying... I'm only going to buy from orchid growers now because I have enough of the type they sell in supermarkets... but those supermarket ones still kept jumping in with my groceries
It's only the last month that I have actually found it easy to walk away from the supermarket ones, and I bet I'll still get the odd one or two more.
If you follow Johns good advice then you can get some good stuff at the supermarket. I just think you can get a wider range and choice from good orchid growers. For a long time I only saw the supermarket ones and I thought all Phals looked like those ones... suddenly looking at a growers site (and here on OB) I realised the stuff you get there is a very limited selection of what actually exists in the world of Phals (same goes for the other genuses you can get at the supermarket).