Well, a couple things to correct: “me gusta esta falda, pero no la quiero comprar” (la falda, therefore feminine “la”, so you were correct in your assumption :))
“Le pediré su número”, is more roughly transated to “I’ll ask for their number”, which like in English might shine some light on how you could be more specific, by providing extra information about the indirect object in question in your sentence to remove uncertainty: “Le pediré su número a él/a ella” (in your two cases)
Like I said, I’m no good when it comes to grammar, but I can tell you, that there are just so many languages, and many have features that others just don’t (like Russian has no article (definite or indefinite), Arabic has verbs that depend on the gender of the speaker, etc). With Spanish I just know that the biggest hurdles are the past tense, and the gerundive, but I can’t really point you to a good resource other than a book I kind of saw a while back: Pons. I read the “german version” (I think) for learning Spanish (I was tutoring at the time), and it was quite informative but dense. Maybe there’s something for you there as well.