Hi, you. If you're reading this and thinking that you want to speak at Litmus Live but you don't feel qualified then I'm speaking directly to you: ignore that feeling.
Look at the speaker lineups for this conference. These speakers have come from all different parts of our industry, with different skills, and very different experiences; that's a huge part of what makes this conference so wonderful! So if you have a story you want to share, dev tricks that you find useful, design data, marketing secrets, whatever— if you have something that you're excited to share then go for it.
My one big piece of advice for actually creating a proposal is to build an outline for your talk before you submit your proposal. It will help you more clearly convey what you want to speak about AND you get to knock out some work for your talk ahead of time (bonus!).