I am a sub that is very new to the BDSM scene. I know a little bit, but I'm quite unfamiliar with a lot. Recently, I met someone online that immediately recognized me as a sub, and I would love to impress him.
He asked me what my limits are, and what I'm willing to do, but I don't know. Is there something I can do to gain some knowledge so that I can tell him?
There's a lot you can do to learn about BDSM and what your limits might be. You've made a good first step by asking for help. I can also assume that you've done some reading here on the site so that you know whether being a submissive is something you really want to do and not just something that this person is convincing you to be.
First, I suggest you learn the basics of BDSM. This means the relative definitions of Bondage, Discipline, Dominance, Submission, Sadism and Masochism. Figure out from those which might interest you to explore; either as the giver or the receiver. Once you understand the level of these core terms and where you might place yourself in them, you can put together the smaller pieces.
Pick up a BDSM checklist and start going through it one by one. A BDSM Checklist is a list of activities, both common and uncommon, that could occur in a relationship or play session. All of them are up for negotiation and many checklists are extremely detailed. Don't stress about knowing every single thing. If an activity in it is foreign to you, look up information about it before rating it. Listen to your gut response. Is it positive or negative? Do you want to leave an activity out altogether? That's fine too. Be honest with yourself.
If you encounter something on the list you are sure you'd never want to try, that's a hard limit. If there's an activity that you might do under certain conditions, that's a soft limit. And even if you read about something and you still aren't sure, that's okay too. You are at the beginning of your exploration and can't be expected to know everything. And, don't be afraid to be honest with yourself. Your likes and dislikes now could change down the road as your experience level grows.
All of this is related to the activities involved in BDSM and D/s relationships, but it doesn't help you set limits for the relationship itself. If you have any experience dating or with previous relationships you probably have a few things you know are deal breakers for you as far as compatibility with a partner. If you are looking for a long term relationship that is face to face make sure your potential partners know this. Are you okay with drinking and smoking or other habits? What does the future look like with you? Do you want children? How important is your career, home life, and social life? Make sure your limits list includes more than what you'll do and not do in the bedroom.
I wish you luck exploring your new sexual identity. Take it slow and learn all you can.