I'm just starting out as self employed, I'm registered with HMRC and I have two business accounts in the correct names.

The question is, when it comes to money, what's what? I know as I'm a sole trader all the money is mine and it all goes on my tax return as my income etc.. how should I account for what's what? Should I be putting money in the business account for the sake of it being there, and then paying for things out of that account when it's for the business?

I am using equipment for this self employment that I already had.. how do I account for that? If I have an invoice/reciept then I can understand it but what about my two year old laptop?

The other big question I have is about allowable expenses and mainly around cars. I have two cars and the primary one gets used for my employment and I claim business mileage back from my employer. I will be using the same car for car shows (the self employment is in the Automotive accessories industry) and demonstration and the like... how can I account for that and what is an allowable expense?

The things I can think of are:

Accessories (Wheels/add-ons etc)
Wear and tear items (tyres etc)
Car loan (although this isn't necessarily clear cut as I might refinance it at a lower rate)

How do I account for these and what is allowable? It sort of makes sense that I claim back a mileage rate but is that going to affect my ability to claim for things like a set of wheels?

I would appreciate any help that I can get.. I don't have an accountant yet but I guess I will need one?

Cheers