How should I use my Oyster Card when travelling from Heathrow?

Last year I bought an Oyster Card while visiting London. But I arrived from Gatwick so I didn't need to use it until I got London Victoria. Anyway, next month I am going to London again and I read that one can top up the card with a "Pay-as-you-go" system. Ok all that sounds comprehensible. I saw also that one can take the tub from Heathrow to London but Heathrow lies Zone 6 and I bought my card on Zone 1. Maybe it isn't important where you buy your card? But when I get to Heathrow underground what should I do? If I want to use the pay-as-you-go should I chose Zones 1-6? How much money is that? Or should I buy a simple travel card get to London and then top up my card with some money? Where I come from we don't have this system. You just chose to pay for 1 day, 7 days, 1 month and so on...Well let me know. I'll be more than grateful! Thank you!