This is a somewhat broader topic, but it applies just as much to BDSM as it does to life. Let's say you want to try something. Maybe it's sky diving. Maybe it's anal play. Maybe it's a massage class. Maybe it's a flogger. Regardless of what it is, you want to try something new. How do you get that to happen?
1. Use I Statements instead of You Statements.
This is something that gets beaten to death in therapy, but they have a good point. I Statements are statements that start with an I - I feel, I wish, I want. They put responsibility on the speaker. They talk about the speaker's needs and desires. By focusing on the speaker rather than on the target, you keep it from being confrontational or about the target. Even if you want something out of the target, you can phrase it in an I Statement.
-I feel sexy when you tie me up.
-I wish you would try these handcuffs next time.
-I want you to spank me.
You Statements are, by nature, confrontational. They put the responsibility and the onus on the target. They put the target "on the spot" and force them to react.
-You should tie me up.
-You should use these handcuffs.
-You should spank me.
They put the target in an awkward place. No one wants that.
2. Be clear about what you want. When you say you want your partner to dominate you, do you mean just tie you up? Do you want to be humiliated? What about role play? Do you want your partner to continue even if you resist (safe words!) or do you want No to always mean No? Be specific!
3. Listen to your partner! Let's say you want to be tied up. The conversation could go something like this:
You: I'd like it if you tied me up next time.
Them: I don't know if I'm comfortable with that.
It's not an uncommon response. But your lover's comfort is incredibly important, so you have to respect her position. Ask follow-up questions: "What about it makes you uncomfortable?" or "Have you tried it before?" And then, listen! Communication and sex are both about give and take.
If you don't know what you want, research! There's erotica. There's porno movies. There's pictures. There's blogs and forums. Start looking around and see what appeals to you. Thanks to the internet, you can be as anonymous as you want, so don't be shy! Start looking for what appeals to you. Approach it from both a scientific and an emotional standpoint - notice your reactions and ask yourself why you're reacting that way. If you're shocked by something you see, is it because you've never seen it before or because it bothers you? If you're interested in something, what about it appeals to you? Is it the fact that the target is getting paddled or the fact that the dom is dressed as a school mistress? Try to come up with specific things you want to try.
Some people have a hard time talking about sex. This is for a number of reasons - social taboo, religious upbringing, naturally shy, etc. It could be the first time you're talking about it with a partner or it could be trying to shake things up in an established pattern. If you're having a hard time talking about it, there are two things I'd recommend:
1. Write it down. This lets you really think about what you want to say and how you want to say it. You can ask your lover to read it without you and get back to you so she has time to process and respond as well.
2. Show an example. Let's say you found a porn video that really appeals to you - why not share it with your lover? Remember to use I Statements - rather than "You should watch this and try it," try "I liked this video and I thought we could try it together."
These communication tips are great for other areas of your life too, not just your sex life. Fighting with your siblings? Same rules apply. Talking to your boss about a raise? Same rules. If you're interested in learning more about how to assertively communicate with someone, the absolute best resource I can recommend is The Assertiveness Workbook: How to Express Your Ideas and Stand Up for Yourself at Work and in Relationships by Randy J. Paterson. Long title, but amazing book about communication. It'll change the way you talk to people about everything and anything.