You need both a card reader and software.
An Omnikey 3121 is probably about the cheapest card reader for driver tachograph cards.
The software, of which there are several programmes, will cost considerably more.
If you really want to keep your own records you may consider buying some tachograph thermal printing paper and doing daily printouts and entering the data into a spreadsheet.
Alternatively you could, after purchasing a card reader, use Tachomaster for £1 per week.