That's a good question. While it would be exciting for you since you now know sign language and would like to "use it on someone else", it can be awkward for the others when they are approached like that.
I've been approached by deaf and hearing people like that.
I was at a restaurant with a few deaf friends. While we were sitting down, this deaf guy approached us and started asking if we were from NTID. When we politely told him that we were, he got all excited and started talking about his past with NTID. He wouldn't stop. His kid started running around, so it made him stop and walk away. When our food arrived, he started talking again. Not wanting to be rude and eat while he was talking to us... we waited for him to stop... which he never did... until his mom stopped him. Whew!
Another time, I was in a book store. While I was looking at a couple of old books, this woman approached me and asked something. I didn't understand her at first and asked her to repeat. She then noticed my hearing aid and was like, "Oh... you're deaf? Wow... awesome! Guess what, I was a teacher and used to work with deaf kids. By the way, I know sign language. It's been years, so I've forgotten some a little... but I still remember. See? I know my ABC's!" *starts fingerspelling the alphabet* Halfway through, she was interrupted by a co-worker asking her for help. I quickly walked away. Whew!
So, in both situations... it was very awkward. Both approaches were very wrong. The topics were also wrong.
In your case, think of it as if you were meeting a hearing person. Would you normally approach a hearing person in the middle of a store or a restaurant and introduce yourself (for anything other than asking them out on a date)? If you wouldn't, then approaching a deaf person should be the same. If you do, then use the same approach. Just don't start signing as if it's so exciting for you and that you're so excited to be meeting a deaf person. That would probably be an insult to them.
I would suggest it be a natural meeting... in a social setting. If it was at a deaf social event like Deaf Coffee Chat or something, you could easily approach a deaf person and introduce yourself like you would with any hearing person.
without teach?? 
