Summarize:

Focus on the things that interest you, and you'll be able to find people who share those interests. Common interests are a good basis for friendship. When talking to someone new, be careful about diving into controversial topics. There is nothing wrong with being interested in topics like politics or religion, but plunging right into them right away can put people off. This is unless, of course, you have joined a group of people with a common perspective on these issues. To make friends, you are going to need to put in a little effort. Call or text the other person, arrange to hang out somewhere outside of where you met. It's okay to be a little pushy. What seems like too much to you as an introvert might be just what the other person is looking for.  Making plans for later is a good way to stay in touch, especially if they are concrete. Even if it doesn't quite work out, it lets other people know you are open to meeting again, and may spur them on to other action.  Be specific when making plans. For example, rather than saying "We should hang out sometime," say "Would you like to see the new Spielberg movie next Saturday afternoon?" This makes it more likely that you'll follow through on your plans. If someone reaches out to you to communicate, return the call. You can wait a little bit before you get back to them, but not returning the call or message is good way to push out those who want to be friends. Refusing to communicate, through phone calls or other means, is not introversion. It could be shyness, or perhaps even depression, but those are not the same thing as introversion. Communicating doesn't have to mean phone calls. Introverts may not always enjoy speaking on the phone because context clues like body language are often missing, and there isn't as much control over the conversation. Text messaging, video chats, and even old-fashioned letter writing are all good ways to keep in touch. Just make sure you and the other person are in agreement about the best ways to communicate. Friendship is a process and takes some time. Allow for some early awkwardness, remembering that it will get easier if you push through it. Even if you aren't sure you're into it right away, fake it until you get past it.
Be yourself. Make contact. Return messages. Use different forms of communication. Be patient.