You can claim tax relief on equipment which you buy for your employment or self employment. A computer, printer, scanner and associated equipment falls into this category.
Step 1. Do you have a receipt? That is ideal but not essential. A payment entry on a bank statement or credit card will often suffice.
Step 2. You need to write down the reasons why you need the computer for work. There has to be some business use.
Step 3. Ideally you will then apportion the cost between business and private use. There was a time when this was probably 50/50 or more towards private use. However these days people tend to use mobiles or tablets for Facebook and photos so an estimate of between 80% and 90% might be reasonable.
A general link is here
More detailed ones are here