Hello and welcome to the site. Thank you for your question.
You need to register with HMRC if you wish to complete a Self Assessment tax return for the first time. This helps to make sure you pay the right amount of tax and National Insurance at the right time. HMRC will issue you with a Unique Taxpayer Reference number (UTR). It is made up of ten numbers, e.g. 12345 67890, and you quote this reference for all future dealings with HMRC.
This link is for you to check if you need to fill in a self assessment tax return.
You need to answer these questions -
1 What was your work status in the tax year you're checking?
2 Was your income over £100,000 in that tax year?
3 If you or your partner claimed Child Benefit, did either of you have income above £50,000?
4 Did you have income from UK property or land?
5 Did you have any other income you need to report?
Based on your answers you would know if you have to fill in a Self Assessment tax return.
As you have income from abroad and also investment incomeyou would be expected to fill in a self assessment tax return.
There would also be a link for you to register for self assessment and other relevant information here.
Now, answers to questions raised by you
Seeing I started working in the UK:
when do I register for paying tax in the UK?
You register for self assessment in order to file a tax return. Your salary will be taxed at source by way of a tax code.
how do I register?
I have explained the process above
do I declare only the UK salary I earn or salary and investment?
As UK resident, you are normally taxed on worldwide income and gains. If you suffer any foreign tax on income/gains abroad, then you claim foreign tax credit relief against that tax suffered. There are special rules for UK residents who are not UK domicile and this is covered on page 23 of RDR1(remittance basis)
will I be taxed twice? Or how will it work?
You are taxed once. Under DTA between UK and other countries, you are not taxed twice on income arising from the same source.
More information on this can be found on Page 9 of HMRC guidance book RDR1 here
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.