Entering and invoicing purchases

Hi everyone

I am very new to all of this, so please forgive my ignorance.

I am a gardener. I want to be able to buy supplies for my jobs (for example, compost), then add those supplies to my clients invoice, as well as keeping a record for tax purposes. 

I have the capture app, which I can use to send receipts to my software, but I get very confused as to how to manage it when it's in my in tray.

Can anyone help me with this?