As someone who has served on conference committees for various conferences, our goal is to create an excellent conference. We are looking for speakers who are going to be part of creating an excellent conference. We have a call for participation that is open to everyone, and we also solicit proposals from people who we think have something of interest to share with our community.
If you're responding to a call for participation, then craft a proposal that will be awesome for that conference. Consider filming yourself giving a talk (even if it's not in front of an audience) to prove that you are a good speaker in addition to having a great proposal.
If you're going to approach a conference organizer to discuss how you could be a speaker there, then ask them what they are looking for in their speakers. You might get more insight into what that particular conference needs to do to be an awesome conference.
If you submit a proposal or approach a conference organizer and are rejected, then listen carefully to the reasons why you were rejected. Use this information to help you determine where you need to focus your efforts to make your next proposal more likely to be accepted. You might learn that your topics are more appropriate for another conference instead of the one that you had originally targeted.
While you're working on this, hone your speaking skills. Look for local opportunities to present, such as meetups or small conferences. Also establish an appropriate presence in your field to show that you know your topic. A blog might be appropriate to show your technical and critical thinking skills. Answering questions on an appropriate Stack Exchange site might be good. Go to conferences and take notes during the presentations to see what it is that makes a good presentation, as well as what makes a bad one, and use that information to help you determine how you can be a great speaker. While you're working on your overall goal of being seen as an expert appropriate for speaking at conferences, set yourself smaller goals that move you in that direction.