According to Webster's, an ally is someone who is "joined with another for a common purpose."
Becoming an ally doesn't have to be difficult--it's just being someone who cares!
By educating and familiarizing yourself with LGBT issues, using inclusive language, and knowing some history, you can begin the process of becoming an ally. Another part of becoming an ally is knowing resources and sharing them with others; perhaps the most important piece is being a safe person to talk to and someone who will stand up and fight for diversity.