Whether the work you do is treated as employment or self-employment is not a matter of choice, it depends on the terms and conditions of your work. It's important to know this because it affects employment and benefit rights, and how you pay tax and National Insurance.
If you're self-employed, you pay Income Tax and National Insurance on what is known as 'taxable income'. Taxable income is mainly the profits you make from working for yourself (whether as a sole trader or partner).
This guide explains Income Tax and National Insurance for the self-employed through a series of short videos, including self-employment: sole trader or partnership? and an introduction to registering for Self Assessment.