Tyres
Pump up your tyres. If they feel soft, get someone to help you pump them up. Check they are not worn down.
Brakes
Test your brakes before every ride and make sure they are not rubbing against the tyres.
Chain
Make sure it is clean and there is no rust, that it is not hanging off and runs smoothly.
Saddle
Make sure it's adjusted to the right height and tightened up. Too high or too low and it won't be comfortable or safe.
Lights and Reflectors
If you are riding your bicycle at night make sure the lights on the front and back of your bike are both working and can be seen. Use your lights when the weather is bad. Remember always wear a safety helmet and follow the Highway Code.