I have been pondering this question for a long time due to certain aspects of my life. I often think of people who get discriminated against because of their age, their gender, their skin color, their religion, their sexual orientation, and their physical or mental ailments. Can HaShem use people like those I mentioned to be His servants? And if so, what can one do to know His voice?

Definitely, God created each of us precisely as He loves us, and we all have a unique and Godly role. We don't have prophecy today to speak with God, but we know what He wants through the details of His 613 commandments, delineated in the Torah. Each of us has a unique spark of God, and one gets to know his "letter in the Torah", especially through mitzvot or Torah teachings with which you feel a special affinity or leaning, which especially excite or activate you.