What can i do to my focus?

Johnka is right of course. You won't actually 'improve' the car only alter it to reflect your taste, good or bad. As said there are numerous sites that offer 'go faster stripes' but few are value for money. Changing the induction and exhaust system is most likely to detract from the performance especially the mpg as Ford have done all the necessary research. Alloy wheels won't improve much apart from increasing the risk of theft. Sorry to be a killjoy but a tarted up Focus will never be anything other than a tarted up Focus. Save your money and buy a hotter version when you can. Much more lasting satisfaction will occur.