Hi, I am a fellow Canadian in Ottawa, Ontario. I am a legal assistant as well, but currently in another area of law. I have previously worked in Business Law and have done some securities work. I find the lawyers are very helpful. Don't be afraid to ask questions! Especially from the associates who are learning themselves. They can be a great resource. The other assistants will also want to help you so you can be part of the team.
They obviously hired you because they thought you could do the job and they must know you are not as experienced in securities. So just go in and enjoy it as you head into this new opportunity. The more varied experience you have in law, the more marketable you will be.
You can find me on my blog: secretaryhelpline.blogspot.com/