You readily admit to not understanding your tax affairs and the tone of your post suggests you do not want to become a tax expert either. This is fine but it means you really should get an accountant to help with this problem.
The only advice I can offer is to retain every single piece of documentation which is even remotely related to your work. As a self employed person you have a certain latitude in what can be classed as a business expense but let your advisor make the decision on what is submitted as expenses. He or she will have experience of what categories are generally accepted and what are classed as taking the p***. The objective of the exercise is to reduce your taxable earnings as much as is legally possible thus leaving you with a small or non existant tax liability.
Choose your adviser carefully as some are more proactive than others. The ideal scenario is that whatever fee is paid to your adviser is returned to you in a reduced liability and/or savings in time and hassle. Good luck with it.