Man, ive been keeping fish for a few years now and still wouldnt keep discus! Have a read about them somewhere! The fact that this woman sold you these fish is a joke. Not your fault buddy, after all she is the one that's supposed to know. Take them all back to the store & get your money back, and put it towards your new bigger tank! You can always buy more again when you have the bigger tank! If i were you, i would stick to guppys or neons now, This is what i started with and are good to help you get the hang of the hobby!
Mind you, neither guppies or neons are hardy enough to cycle a tank these days.
Somebody else suggested rosy barbs- too big for a 15 US gallon (12 imperial gal) tank. My neighbour's rosies are easily 5" long and v. active- they need lots of space.
Please, let's think about the actual tank situation before we just toss out suggestions of fish.
Fishdisease, you need to take those fish back to the shop. She has mis-sold you those fish, point it out pleasantly but firmly quoting adult size of the fish, and you should get a refund.
Then you need to read up the pinned topics about cycling a tank and think about whether you are going to start again with a fishless cycle or cycle with fish. If the latter, you will be limited to a few hardy kinds: danios, platies or black widow tetras are the ones usually recommended; given that your tank is not very long, I would suggest one of the latter (though the fishless cycle is better!).
Basically, your tank is suitable for keeping about 7-8 fish with an adult size of not more than 2", or 4-5 3" fish. We are talking tetras, platies, guppies (if doing fishless cycle or introducing after cycle), corydoras- plenty to choose from but check how big they grow. Also remember that many fish need to be kept in schools, this you need to respect. So if you are getting tetras you need 5-6 of them, corys probably 3-4. A schooling fish kept on its own will suffer permanent stress that may damage its health. So this obviously affects what you can fit in. You should avoid the fast swimmers, like swordtails and danios- they need a longer tank.