Hola Mela. We're going to have to stop meeting like this :D .
I think I can shed some light on "gustar." Even though we translate "me gustan los gatos" as "I like cats," it might be more useful to think of the sentence more literally: to me are pleasing the cats (or, the cats are pleasing to me--hence, I like cats). "Gustar" really means "to please," and the subject is not the pronoun that precedes the verb (me, te, nos, etc.) but rather what comes after the verb (in this case, los gatos). And the verb here is "gustan" rather than "gusta" because it has to agree with the subject, which is "los gatos" (plural).
As for a mí, often that's optional. It's added for emphasis or clarity or just out of habit, but it's often not necessary. It's just as correct to say "Me gustan los gatos."
In the second sentence, "A nosotros nos gusta las camisas," I think there's an error. It should say "A nosotros nos _gustan_ las camisas." Here, the subject is "las camisas" (the shirts). The shirts are pleasing to us (i.e., we like the shirts). Thus, the verb is "gustan," to agree with "las camisas." (If you used "gustamos," the sentence would make no sense: to us we are pleasing the shirts :roll: ) Again, "a nosotros" is optional.
By the way, if the second sentence had said "A nosotros nos gusta la camisa," it would have been OK. If you change "shirts" to "shirt," then you need a singular verb (gusta).
I might note that while "gustar" is the best known of the verbs that work this way, there are others as well, so it's worth spending some time to understand how "gustar" works. You'll need the same principle for encantar (to like very much), faltar (to be missing, lack), importar (to matter), interesar (to be interested in), parecer (to seem), and others.
One more thing--overwhelmingly, "gustar" is used in the third person singular (gusta) and plural (gustan). However, occasionally you'll hear it in the first or second person. There's a well-known song by Manu Chau called "Me gustas tu" (you are pleasing to me--in other words, I like you). You can find it on YouTube: __http://www.youtube.com/watch?v=mzgjiPBCsss__
I hope this helps.