Hi, I'm self-employed and about to start some paid employment too. How do I categorise employment income in transactions?

Hi thalia2 Thanks for reaching out to us here on the Community. 
You could create an asset account within the chart of accounts; https://quickbooks.intuit.com/learn-support/en-uk/help-article/chart-accounts/add-account-chart-accounts-quickbooks-online/L0mCv3AKn_GB_en_GB?uid=m2uh8ujh and allocate any income/credits from your paid employment to that asset account by creating a bank deposit; https://quickbooks.intuit.com/learn-support/en-uk/help-article/bank-deposits/record-make-bank-deposits-quickbooks-online/L2BBZOPdr_GB_en_GB?uid=m2uhi7ca

Select the +New > bank deposit > in the account field select the bank account you received the amount to > In the date field enter the date you received the amount into your bank account > in the 'add funds to this deposit' section enter the asset account that you created in the account column > enter the amount that you received > in the amounts are field select No VAT > save and close > you'll be able to match this deposit to the amount that you received in the banking section. Using this method would mean that the amounts wouldn't show as income on your Profit & Loss or be used to calculate the figures within the income tax section. 
Please note that we're not accountants and that we recommend that you verify the above with an accountant or there maybe an accountant, or suitably qualified Community member, who reads this post and they're able to confirm our advise above is correct or recommend an alternative method.
If you require any further assistance or information from us please reach out to us and we'll be happy to assist you further.