There's a few issues going on there.
I would add my support to the notion of buying your own test kit - that way, you can test what you want, when you want, and don't ahve to rely on the LFS being open, and the staff member you are served by being competent to interpret the results correctly.
Have a read of the green link in my signature area, it deals specifically with the situation you find yourself in. You really need to be doing water changes every day to keep the ammonia and nitrite as low as possible.
I would add that none of the fish you have selected are really suitable for a 40l aquarium. This is why your ammonia and nitrite have built up so quickly - you have a lot of fish in a small tank. Have a read of
this thread, these are all fish which are suitable for your tank - I would suggest as your fish naturally die off, you replace them using that thread as a guide.
And if you have any more queries, however trivial you might think they are, please just post them - we were all beginners once, and some of us are young enough to remember what it was like!