How should I get the best price Tube + train fares?

A National Rail card is your 16-25 railcard - remember this cannot be used on tfl trains. If you are not in London often a travel card might be a better option than an Oyster Card. You have to 'top up' those and make application for one, but a travel card can be brought when you arrive. However, remember London is zoned for travel purposes and you will need to get a card covering the zones you will be using - there is a map at all underground stations, and one can be viewed on the tfl website. You do not need an Oyster card + a travel card. It is one or the other.