If you are arriving by train into a mainline station the staff are used to explaining the system.
I did have an Oyster card from years ago, I actually gave it away. These days the only reason to use it is so that you use the correct card to tap in and tap out. But you can do it with a credit or debit card, but use the same card.
The cost will be capped to the daily maximum, so if you take three journeys on the same day you won't pay any more that day.
Tapping in and out is done at the automatic gates but if you have a lot of luggage or a pram there is a wider gate the staff can open. Very similar to most train stations these days.
Once you have tapped in follow the signs to the line you want and the direction you want.
There could be escalators (stand on the right) lifts, stairs or level access, just follow the signs.
Once on the platform there will be a sign saying how long the next train is, just wait until it arrives, the doors will open and generally people let passengers off before getting on.
There are help call point on most if not all platforms.
Keep your bag in front of you.
Remember to 'tap out' on the same card.
Oh and London also has busses and trams. Sometimes the bus is a better route and you get to see things.
Have fun.