It's okay to ask my how my transition is going! That's probably the way you should phrase it. This is a big part of my life, and one I don't have a lot of opportunities to talk about in detail - if you're genuinely curious, I'll probably appreciate the chance to talk about it.
It's not okay to give me advice on my transition, unless I've directly asked for it. Even if we're already talking about transitioning in general, it's just weird for you to start giving me medical advice. And if your advice is "wait until you're sure," it's not just weird, it's rude and hurtful. It's not like I'm going to drop by the mastectomy store and get my tits lopped off on a whim, like somebody getting a misspelled Chinese tattoo after a night out drinking. "Wait until you're sure" almost always means "wait until I am okay with it," which in turn pretty much means "wait forever." And that's not okay.
Along the same lines, don't suggest alternatives to transitioning. This one really baffles me - you're really not very likely to be suggesting something new. Or, if you do find something I haven't tried before, there's probably a reason I haven't considered it. For example, a well meaning friend suggested yoga to me, as an alternative to top surgery. It's true, I haven't tried yoga! And maybe I could learn to transcend my body. But, it's not like it will make my tits invisible - I'd have to keep binding, or people would still be calling me ma'am. "Try herbal tea before you try testosterone" is just another way to say "wait until I am okay with it," only it involves even less respect for my intelligence.
But, it's okay to ask me to answer your questions. Note the phrasing: please don't just start the conversation with "how do trans people have sex?" - start by asking me if I'll answer questions, and understand that I may not always be in the mood to do so. Also please make sure that your questions are actually questions - "Why not try hypnotism?" is just advice with a question mark on it. But in general, I want to help people understand. As long as you're making an effort, I'm not going to be offended that you don't know something, and I'd much rather have the chance to tell you than leave you in the dark.
It's not okay to mess up my pronouns. Yeah, when I correct you I'll probably tell you that it's okay, that I know it takes time to get used to it -- but what I really mean is that it's not okay, but I've forgiven you anyway. If you're having a really hard time, practice. I'm an understanding sort of person, but it hurts when you make a mistake, every single time. And I'm getting tired of pretending that it doesn't.
And finally, the most widely-applicable piece of advice: if I know I want to do something awesome, but I tell you I'm nervous about it, back me up. If I hadn't had so many people tell me not to cut my hair, I probably would've gotten a buzz cut at 15 and never looked back. Instead, I spent years going from waist-length, to shoulder-length, to chin-length, to a little bob, to a pixie cut - and at each stage, everybody I asked said I shouldn't do it. In retrospect, I'm a little disappointed in the sheer number of people who should have known better who reinforced mainstream expectations of femininity. People who never wore heels, or skirts, or jewelry, or makeup, or contacts, etc. nonetheless told me that I should keep wearing these things, even when I confessed that I hated them, because they made me so pretty. "Conforms to societal expectations" is not always the most important requirement for your friends' personal choices. Come on, be more awesome than that! Do things just because they're fantastic, and encourage your friends to do the same.
Anyway, because I'm in a little bit of a "trans 101" mood with this post, now is a great time to ask questions! You can argue with me, I guess, or ask anything that's been on your mind regarding trans issues. I'm pretty much never going to be offended by a question that's asked out of genuine curiosity, so don't be shy!