Most illustrators will have started out as amateur artists or have some formal training in their chosen profession. No matter what you do, your first paid job is likely to be a scary one, but also a very exciting time. All your training has paid off, someone has taken a liking to your work, and now you're going to get something published / printed / produced for everyone to see. So what do you do now?
When I got my first professional illustration job, I was floating on a cloud and then fear suddenly set in. What if the drawings came out all wrong? I realized I also didn't know anything about the subject matter. How was I going to draw this successfully? It seemed like a daunting task ahead.
RELAX! If you feel like this, it's natural. You WILL be able to do whatever job you have been given. Each job you get will feel like a stretch - pulling you a little out of your comfort zone. But this is a good thing. Every illustrator feels like this at a start of a new job. You're on a journey to create something amazing.
The first thing I'd recommend would be to get focused about the brief you've been given. Often the client will give you a set of verbal instructions rather than the kind of typed brief you may have got in illustration school. Even the best human memories can forget often important details. So as soon as you get out side of the meeting, make some notes as soon as you can, while the details are still fresh in your mind.
*TIP* Also establish when the next meeting is, overall project costs, what you are expected to have done for your next meeting, and what the overall time scale is.
Unless you're lucky, the subject you're going to be illustrating may be completely alien to you. Don't panic if you've no idea what some of the things you're going to be illustrating look like, the next stage is to do your research. This is where you get as many visual references of the things you're going to be drawing as possible. Become an expert in every element that's going to be in your picture. As well as the local library, use Google's Image Library - http://www.google.com/imghp - and start to print out or save images of the kinds of things you're going to have to visualize. Even fantasy work will require a certain amount of research. This can often be a lot of fun, as you learn about new things.
TIP: Always ask the client if they have any books or photocopies you could also borrow. If it's an educational project, it's likely they've already done a lot of the research you would have to do anyway. This can be a great time saver.
Make sure you also know the specifics of the drawings you'll be creating. Obvious things like color and size need to be considered. Clients are often vague about the dimensions of the illustrations they require, but try to press them on this. I once had to redraw 70 illustrations just because I didn't get the exact dimensions of the drawings that were required. I learnt a hard lesson there!
Get a cardboard folder and start putting any research and drawings you do in there. Every project should have it's own folder.
The next thing I do is to start with some tiny scribbles on my note pad. At this stage I'm thinking about rough layouts, and how to fit in the actors in my pictures along with any background settings. On either fantasy or educational projects, you'll likely have been given a framework of what needs to be happening in that scene. Start off by working out who is in the picture and what goes where.
*TIP* Always put something of yourself into each picture and do it in your own style. The client saw the work in your portfolio - and that's why they hired you. Don't do the kinds of drawings that everyone else can do. It's unlikely you'll make your fortune that way. Make it unique!