Make sure your bike is the right size for you - if it's too big or small, it could be hard to control.
Check the air pressure of your tyres, and make sure they're not worn out. Your chain must be well oiled and not hanging off. Check your brakes.
Make sure your lights and reflectors are clean, and that the lights are working and the batteries are charged.
At night your cycle must have white front and red back lights lit, and a red rear reflector.
It must also have amber pedal reflectors.
And remember - if you're not sure about anything, check at a bike shop.