You can claim back legitimate expenses for your business against tax. (ie pay for them out of untaxed income) So any paper, pens, scalpel blades, computers, post, envelopes, printing, business miles, etc. Also if you work from home an allowance for extra expenses (more electricity etc.). So your turnover might be £50,000 and your expenses may be £15,000 so you will only be taxed on £35,000.
If you are registered for VAT you can claim back the VAT on business purchases against any VAT you charge. So in a VAT period where you have charged out £100 in VAT you may have been charged £50 in VAT and so would pay the VATman £50.00. But likewise if you have charged out only £50 and been charged £100 you will get back £50.00.