In reply to SophieEmily:
What is your budget? How far are you planning to cycle? What is your previous cycling experience? Will you be cycling on or off road? Do you plan to cycle as fast as you can, or to pootle along? Do you want to wear special gear to cycle and shower at work, or do you plan to cycle in work clothes? Do you have to carry any kit on your new bike? Do you plan to cycle all year around in all weathers?
You can make any bike look not worth stealing with an ugly paint job...
http://www.instructables.com/id/Anti-Theft-Bike-Painting/