Nearly 14k for a six year old C class Merc, rather you than me.
If you really are rubbish at buying cars then some simple steps to get a few ideas are:
How big? So Fiesta size, Focus size or Mondeo size, do you want or need 2,3,4,5 doors etc, do you want an otherwise normal size car that is taller because you can’t drive properly?
Are there any makes you particularly like, eg Merc, Audi, BMW or ones you want to avoid eg Vauxhall and Peugeot
These two criteria alone should help you get shortlist of models. Then take a look at that model on parkers.co.uk or similar for a list of the different specification on that model, eg trim and engine. You can get a load of information on performance and economy. At some point you’ll want to consider fuel type too. Diesel offer better economy but only if you do a fair number of longer journeys and cost more to buy generally, Petrols tend to be better for shorter journeys and can be better to drive, though this is generalising a bit.
Once you have settled on a car and narrowed down the specification a bit get on auto trader or piston heads and start looking
Having said all this this is STW so just buy a SEAT Leon FR because its what I drive and is really great, obviously.