I moved to the UK in August 2018 on a spouse visa.
To date I've made less than £6000 which hasn't been taxed as I am under the earnings threshold.
.....I got married last year,
Do I need to do a self assessment? What forms do I use to file in the USA?
Start with the UK and HMRC.
The UK tax year is from 6 April to 5 April of the following year. You have UK earned income in 2018 and will earn more in 2019. Your first tax year will be 2018/19.
First, you have 2 options as to how you file your UK tax - the Arising basis and the Remittance basis. Since you have indicated no other income from outside the UK
after arriving in the UK, you will most likely select the Arising basis. If you continue to have no income from outside the UK, you will be solely under PAYE (Pay As You Earn) and not self assessment. The threshold for tax liability for 2018/19 is £11,850. The £11,850 is your Personal Allowance. If you surpass the Personal Allowance, HMRC will automatically withhold (through your employer) tax which will be due for 2018/19. As a result, you would not have to file any tax return for the UK for 2018/19. A majority of UK employees do not file a tax return.
For the US and the IRS.
Your first tax year residing abroad will be 2018. Since you were married on 31 December, you will file as married.
Several decisions must be established first. The first decision is do you file MFJ (married filing jointly) or MFS (married filing separate). For MFJ, do you want your spouse to become subject to all IRS reporting requirements and informational obligations? That would mean declaring all their income, both earned and unearned, for 2018,
worldwide, plus an informational obligation for any investments. They would also have to secure a TIN (Tax Identification Number) with the IRS which may require special use of a qualified individual. For these reasons, most file MFS which avoids your spouse becoming subject to the IRS and would include only your income and investments for 2018.
At this point the standard basic kit for filing, in your case using MFS, would be:
1040
Schedule B
Schedule 6
Since you were resident in the US for part of 2018, you may also have form 8965.
The second decision:
Will you take the FTC option (Foreign Tax Credits form 1116) or FEIE (Foreign Earned Income Exclusion form 2555)?
Since you will likely have no UK tax paid in the 2018 US tax year, or possibly for the UK tax year 2018/19 if you remain under the Personal Allowance, FTC may not be a consideration for 2018. So, on to FEIE. To file 2555, you will need, at minimum, a requirement to be tax resident in the UK for 330 days. If you do not wish to pay tax on your 2018 UK earnings to the IRS by April17(?), 2019, you will need to file for an extension to use 2555, in your case to 15 October 2019, for your 2018 US reporting. Once you have 330 days in the UK, you may file.
If you select the FEIE method, you will have 2 additional forms required:
Form 2555 or 2555EZ
Schedule 1
In all cases, it is likely you will continue to use the standard deduction.
And, if required, don't forget to file an FBAR by 15 October 2019.
Additional decisions:
Will you use tax software, IRS FreeFile, IRS fillable forms downloaded, or a tax preparer to complete the forms for you. If on your own, will you file electronically or by mail or courier.