I'll be in London for a month only and need help with transport info?

I don't think I'll be using tube or bus every single day but often so i want to know what kind of oyster will be appropriate. i.e: how much will i be charged if i make two journeys one day. whats is that daily capping thing? Also, would it be better to get payg with maybe �40 and buy single tickets to go from and to heathrow? pls help