Why is it "el" for guardarropa when it ends in an "a"? Why is it "el" for sofá? Are they "irregular"?? I'm sure my teacher told us but I think I didn't understand or didn't hear it.
Again, I thank anyone who helps me. :) Gracias.

Not all nouns follow the 'rules' we've been taught to help us to guess the gender. My advice: memorize the gender along with the noun each and every time, instead of the 'rules' which don't always work.

Along the same line, some nouns end in an s, and are both singular and plural.
For example:
el parabrisas (the windshield), los parabrisas (the windshields)

As Rusty says, there are exceptions to the general rule. But there are some other things that can clue you in as well. For example, most compound nouns-- "guardarropa", "cubrepolvo", "limpiaparabrisas" etc. are all masculine, regardless of the gender of the component words.

A very tricky case is the word 'problema', which is masculine, maybe because feminine in spanish entails some kind of sympathy, it doesn't match trouble.
English speakers often say 'no problemo' (mistake, we say 'sin problema(s))
Another example, the word 'mar' (sea)
Is masculine if you don't live by the seaside and see it as something distant
Is feminine if you live nearby the sea. Fishermen say 'la mar'
Funky language, it is, but German and French also have weird gender assignments

Problema is one of a class of words that come from Greek and end in -ma and are masculine in Spanish:

problema
tema
esperma
esquema
lema
sistema
telegrama

thanks everyone for all this info and help! :) :)

Perhaps in origin was masculine
el armario para guardar la ropa --> el armario guardarropa --> el guardarropa.
It's only a guess :D

Same in English, one half of the compound calls the shots, not both....