The main difference between "un" and "una" is that un is masculine and una is feminine so you have to use un or una depending on the word, if the word is masculine like "ordenador", "anillo" "amigo" you have to use un, but if the word is feminine like "letra" "amiga" "profesora" you have to use una, so you fave to focus on the word below.
I think it's important remind you when to use un/una in a sentence instead of el/la:
We use el/la to refer to something specifically: "El perro de mi tía" you are talking about one dog specifically, your aunt's dog.
However, we use un/una to talk about something not specifically. Look at this example:
-¿Qué quieres beber?
In this case we would say "un café" cause we want a coffe, we don't mind which coffe, it' would be stupid to ask for a coffe specifically like "give me this coffe" you want a coffe and that's it
I hope you have understood it with my silly examples hahaha but I think that jonking it's a goos way to learn.