I'd agree with all the advice given so far, particularly the often-repeated advice to own up to any mistakes. They will never be as bad as you fear, and telling someone early may make a huge difference to how easy it is to fix.
Things I look for/value in trainees:
- Attention to detail. Nobody is expecting you to be a legal genius or an expert negotiator; you simply don't have the experience. However, there is absolutely no excuse for spelling/grammar mistakes or careless errors. If you're asked to proof-read/check something, do a really good job. If you are a "safe pair of hands" with the relatively mundane work, then it will stand you in fantastic stead going forward as the more senior lawyers will rely on you more and more, which in turn will expose you to more challenging work. If an associate has to constantly check your work to correct errors, they may as well do the work themselves which is massively inefficient.
- Willingness. Don't complain about being too busy. Always smile and appear enthusiastic when asked to do work, even if you are (politely) suggesting that others may have more capacity.
- Timekeeping. Trainees who think it's ok to turn up late in the morning because they were working late will not last long. Occassionally you may be told that you can come in late if its been a particularly gruelling night/series of nights, but never assume. If a team is busy, that is exactly when they will rely on you to be punctual in the morning.
I agree with those who have suggested working for as many partners/seniors as possible. Not only does it expose you to a variety of work, but it also ensures you have a wide pool of potential "appraisers" come evaluation time. This will be viewed positively if you're ever up for promotion to the next level.
Re. maternity leave, I'd suggest waiting until you're at least 3/4 years qualified, unless you are very confident about your "profile" at the firm. 2 years PQE is a very popular time for associates to drop out or move to another firm, and at that stage you are still fairly expendible.
Think carefully about your specialty. Not only are certain areas of law more "family friendly", but several (pensions, incentives, insurance etc) are so specialist that they will give you a greater bargaining position re. salaries. Corporate and banking are more mainstream, so the lawyers are considered to be more expendible. I really wish I'd known that before I picked my qualification area! Conversely, certain specialities (IT/IP, employment, commercial) lend themselves well to an in-house role, so think about whether this is a route you may go down.
Above all, the learning curve as a trainee/NQ is incredibly steep - you will learn something new every single day, and life will get easier. So don't panic if it all seems overwhelming to start with.
Finally - if you are any good with maths, accounting or Excel, you will be highly valued. Lawyers are generally rubbish at these, and will stare at you in wonder and amazement!
I'm currently on my second maternity leave from a US firm, having qualified in a silver circle firm. It's hard work, but (generally) I really do love my job. You'll get out what you put in. Good luck!
Sorry - that was rather more of an essay than I intended!