You can add money to an Oyster card or use contactless (card or device) straight away.
You only pay for the journeys you make and it's cheaper than buying a paper single or return ticket (train companies may offer special deals on some journeys).
In Zones 1-9, your fare is capped so you can travel as much as you like in one day or week (Monday to Sunday), without paying more.
You can only use contactless to pay as you go on TfL Rail to or from Reading to Iver. Journeys to or from these stations aren't included in caps.
Top up online, see your journey and payment history and apply for refunds