I'm introverted as well, and it's visible if you hang around me for any length of time. Social events need to be taken on *my* terms, and not sprung on me, otherwise I bristle and back away. Pair that with the fact that I detest casual conversations, makes me a bad person to just happen across and say, "how's the weather," to.
Once you know your limits, you can at least play to your strengths and work on your weaknesses. For me, it's initial eye contact and saying what's on my mind rather than being shy about it and just not saying much at all. My hardest part is simply the immediate icebreaker, but after that I'm good conversation and a good listener.
But it otherwise just takes practice to learn some lines and proper greetings. Sometimes it's all you can do to make yourself be approachable and let someone else make the first (technicall second) move. Give a smile and direct eye contact that stays longer than a quick glance, but shorter than an intense stare; maybe a nod of the head; you don't need to walk towards or face her, but make an attempt to square the shoulders towards her. If they respond with a smile, that's at least inviting some sort of comment, and not showing that they think you're a creep and hope you're just walking by.
Comment starters are best made about something you have in common. Maybe she's wearing something you genuinely like and you happen to know about it beyond that she looks sexy in it, like materials or labels or just the look in general. Worst case, you're both standing in the same physical area, so you do have that in common (grocery store vegetable aisle? movie ticket line? burger king? parking lot? bar with music playing?) Just try to not ask yes/no questions. And yes, that takes practice to think on your feet before you ask one.
Practice with people who aren't your type. Older ladies, women with rings, even guys. The pressure is clearly off, and they likely are fine with it.
I get the, "I don't care" deal. There are topics I don't care about, or conversations I don't care about. For those, you just have to steer it elsewhere, move on somewhere else, or just admit your ignorance or lack of exposure (not that you don't care, but that blank look of yours can be explained as just never having thought about it before). Ultimately, though, you're going to have to care about what they're saying and listen, so you can respond in kind. You might respond with a salutation and move on, but at least that's a positive interaction for both of you.
Lastly, not all comments and social interactions need to lead to deep conversation, especially if done in a place you frequent or you've seen someone else at a lot. A few hi's here and there and you can open up, "I've seen you around here a bunch, do you work near here..?"
Now I need some therapy...time to go get some groceries and say hi to 5 people as I do.
