[neilm]

The most important thing: don't worry about it.

You will probably feel overwhelmed on your first day, probably in your first week too. That's normal. It takes time to get used to how the company works, how the codebase is structured, who's responsible for what in the organisational structure.

If it's a good company, there should be someone who you will be able to ask anything you need, no matter how often you need it. I've run multiple projects with perhaps a dozen programmers, and I try to make it very clear to new recruits that they are never bothering me. They don't usually believe me at first, but I mean it! Any significant codebase has a huge amount of knowledge and structure known only to its developers, and new guys save so much time if they can just ask a quick question than dig around for themselves. So don't be afraid to ask questions incessantly. Definitely form your question sensibly, but don't sit around feeling stuck.

Honestly, I feel the most important thing you can learn in your first day/week is who is the best person to ask about each part of the code that you'll be using, and a bit about the general structure of the code. You shouldn't expected to produce anything straight away.

Also, have fun! I still really enjoy programming after doing it full time for 15 years and hopefully you will too!