Station entrances
Some stations have buildings where you can enter. If the station has multiple lines, there could be different entrances for different lines.
The design of station buildings is a huge topic and quite fascinating.
Some stations do not have a surface building, simply steps down to a ticket hall:
Some stations have a one way entry/exit system that you must obey (eg Oxford Circus).
Eventually, you will find yourself in some kind of ticket hall.
There will be ticket machines to buy or charge Oyster cards.
There will be barriers to enter the system (a few stations are open, but you still tap in and tap out at a machine). Many stations have wider barriers for luggage etc.
Using the barrier
Find a barrier with a green arrow.
Place your payment method flat on the yellow reader on the right and the gates will open. The display will show any remaining credit if that’s appropriate.
If you have a paper ticket, there is a slot for it (face upwards). Remember to retrieve the ticket as you will need it later.
If you have problems, someone will be there to help you.