Very Stupid Mistake When I Was Under 18

Jacky2019

Okay so, going back about a year and a half ago, I was dumb and desperate for a new computer.
I signed up for finance through v12 retail finance for a new computer totaling around £1700.

I never paid a penny of this back, I couldn't afford too. I was receiving letters up until Feburary just gone, phoned them and told them I wasn't 18 and to stop pestering me because I didn't know what else to do.

Come my 18th birthday, I received a letter saying I owe them the said amount. I haven't made contact in regards to this since as I have no idea what to do.
It is showing up on my credit file/clear score account as "in default" and last updated "9th April 2019"

Another mistake I made was taking out a sim only contract with Vodafone which I lost the sim for and can't remember anything to do with the contract, I used a fake date of birth for this too, I've missed the last 2 payments (Paid for around 2-3 months) and have received letters from debt collectors about this.

Can anyone offer me any advice as to how to fix this please? My credit score is 0, I do eventually want to be able to get a mortgage, car on finance etc but from looking at how it is now , that's never going to be possible.

Thanks