First. You record every single penny you spend on the car, fuel, servicing, tyres, insurance, road tax, AA/RAC etc. You keep tabs on the mileage you are doing so you can apportion an average % of private use based on mileage and disallow that proportion of the total costs of the car in the year.
Second. You keep a detailed note of your business miles in your diary and charge your business 40p per mile for the first 10,000 miles in a year and 25p per mile thereafter.
The best way will depend on your use of the vehicle but 90% of the time the mileage rate works out much better for you as this is deductible in the business and tax free in your hands.
Computers are now generally written off to repairs in the year they are bought as they have little value after a few months of use. This way the whole cost is allowed against profits in the year you buy it.
I hope this helps but don't hesitate to ask if I haven't explained it clearly enough - it is rather late
