I am a student in the UK who is doing some social marketing for a local company and have been told to make a monthly invoice. How do I pay taxes on the money I receive? I don't want to do anything illegal but I have absolutely no idea how to pay taxes on money that is going to be direct debited into my account.
Firstly, keep a record of everything that has happened, finance-wise. This is absolutely critical and you could end up being audited and potentially fined very hefty amounts of money for bad accounting practices.
Next, go to https://www.gov.uk/topic/personal-tax/self-assessment and register as self-employed, to allow you to fill in a self-assessment tax return. This process could take quite some time.
Unless you are trading outside of the UK or earning more than around £90k/year, you don't need to charge VAT on invoices. If you do, you will need to register for VATMOSS via gov.uk's portal where you can do your tax return.
Actually filing your tax return
You do not need to fill in your tax return for the year until the year has actually ended. The date that the financial year in the UK ends is April 5th.
Keep every invoice you've ever gotten, and use these to total up your earnings. If possible, keep a list of expenses, and also have to hand your payslips, P45 or P60 for the year. The site will guide you through the rest.
Also, gov.uk is revamping the self-assessment returns process, so next financial year may actually be easier than normal.