Kittens
Kittens are adorable and it is tempting to take one home without thinking of the consequences. Remember, kittens can be demanding and taking one on means you are committing to lifelong care – perhaps twenty years or more.
 
There are many websites available to help prepare for your new arrival, feeding, training etc., here a few for you to start with: RSPCA; PDSA; Blue Cross and a general web search will bring up many more for you to read.

Rescue cats
Rescue centres strive to rehome pets that for various reasons find themselves without someone to love and care for them. Their owners may have died, may longer be able to afford them, have moved to a no-pet home or had a baby. In extreme circumstances they have been abandoned or cruelly treated. They range from kittens to older cats.
There are many Rescue centres, some big names you will recognise  RSPCA; Blue Cross;  Battersea Dogs home (yes they have cats as well), some smaller local ones like Heathlands (Royston), Luna (Hertfordshire) to name but a few.