It's a fact of Cambridge life, bikes are the best way to get around. Even though Newnham is pretty central, I would definitely recommend a bike to save you time and in case you need to visit a college that's further out.
However, to stay in one piece and prevent your bike from being stolen, follow these top tips:
You can bring your own bike from home if you want to (probably easier as there is an extreme shortage of bikes for sale in the first few weeks of the academic year), but whatever you do, don't bring a brand new expensive bike! Bikes with baskets are particularly great for putting shopping and work in.
Wear a helmet: this is so important, as people can and do get seriously injured on Cambridge's roads (although this is rare).
Familiarise yourself with the highway code and general bike rules such as hand signals.
Make sure your bike has lights! The police carry out regular checks at night and you will get fined if you don't have them on. Reflectors are also a good idea.
Be aware of what's going on around you - always look before changing direction etc and remember those cars and pedestrians!
Trouser clips are a good and worthwhile investment (although they are probably not the most attractive accessories in the world, they DO protect your trousers!)
Familiarise yourself with Cambridge's roads: cycle paths, where you can/can't cycle etc.
Invest in a good lock (D-lock or another strong lock). Cambridge has a VERY high rate of bike theft and it will be gone in minutes if it's not locked! Always lock your bike by the frame or back wheel. Also remember to take your bike lights off your bike whenever you lock it up, as these often get stolen.
Be careful where you lock your bike. Some places don't allow it so always check for signs.
Make sure you paint your security number on your bike at the beginning of term. Numbers are obtained from the Porters' Lodge when you register your bike. This way it is easier to track your bike if it has been stolen.