Thanks for all help. I'm a foreigner and can't immediately dismiss one word as nonsense. Keeping this in mind, the real important thing I want to learn is to know what to listen for, when distinguishing the -e -y -ę endings in normal speech.
I'm happy to detect word boundaries at all at this stage :) but as I said: this will likely get better as I progress in the language.
"koszula" is the basic, Nominative form that you will find in dictionaries.
"koszulę" would be Accusative, used for example for "We have a shirt".
When a sentence using Accusative gets negated, Accusative turns into Genitive. Genitive of "koszula" is "koszuli": Nie mamy koszuli.
It's not the only use of Genitive, of course, but one of the most important ones.
It depends on the phonetics. You can check here: http://mowicpopolsku.com/polish-grammar/cases/genitive/#noun-singular-feminine
"telewizji" is the correct form, not what they put there, also most of neuter singular is wrong, but regardless, it's still a very good site.