Request money or invoice?

[PCHelper]

Hi,

I was previously told that unless I make £1000 or above per year from my own work, I won't need to pay taxes.

Should I still use invoices when people pay me for fixing their computer, making a 3D model for them or 3D printing something for them or other stuff?

Will it be more difficult to pay taxes for this later if I use money requests and exceed the £1000 treshold?