Ok, my sister is a straight play actress and is going to Tisch School of the Arts at NYU this fall, so I've picked up a few things during her career.
A. For sure small (ish) parts are eligible. There have been plenty of small parts my sister has played, and it didn't diminish her reputation as an actress at all. Main roles are the main part of the show of course, but as a small part you keep the show together. It doesn't matter the size of the role, it matters how well you portray that role.
B. It's a pretty simples answer. First off, it depends on what show it is. Maybe you could enlighten us a bit about what the show is? And secondly, all it takes is this: Think back to any show you've seen before, it would be especially nice if you've seen this play before. Now, remember what you liked about the small parts in the show. Don't steal the show, as that isn't your job, but act as best as you can. Get a feeling of who the character is, maybe find a background story for him/her. Then when you are rehearsing your lines, remember who the character is as a person. Is the person flamboyant, outgoing, what? Think of what your character would do in that situation, and what kind of emotions they would express.
Just remember, all you can do it your best. You do that, and you'll be great, I guarantee you'll shine.