Working Tax Credit is based on the hours you work and get paid for, or expect to be paid for. It doesn't matter if you're employed or self-employed, but unpaid work doesn't count as work when claiming tax credits.
You need to be in paid work at the time you make your claim, or starting paid work within seven days of making your claim. You will only get Working Tax Credit if you actually start this paid work.
If you're already getting Child Tax Credit you should call the Tax Credit Helpline to claim Working Tax Credit.
If you don’t already get tax credits you have to fill in a claim form to claim. You can only get a claim pack from the Tax Credit Helpline. You can't claim online.