Look at a map, or Google maps. Take a note of what is the nearest tube station to your starting point. Then note which is the nearest tube station to where you want to go.
Then either (a) look at a Tube Map, online, in a book, on a tube station wall, and look and see which line(s) join the two stations you want, look at the end station on that line to make sure you are going in the right direction. If you have to change you will have to note the line and end station for each.
Or (b) download the TfL app, click on 'Where to?', put in where you are starting and where you want to go and when, and it will give you instructions on how to do it, possibly with a couple of options for speed, or step-free, or fewest changes.
On your journey, as PP said, tap your payment method on the yellow circle at the gate and it will magically open and let you in. To get out at your destination, do the same, making sure you use the same payment method.
On the train there are maps of the line you are on, and, on some lines, lines which run on the same tracks, so you can check your progress. Should you find you are actually going in the wrong direction, get off at the next stop, cross to the other platform (slog up the stairs and down again in many stations) and go back the opposite way.
Have your payment method ready before you approach the barrier. If you do not, don't stop in front of the barrier while you dig it out, or you will cause havoc. Instead, step to one side while you search. Other protocol, on escalators, stand to the right and keep in. People tear up and down on the left and will not hide their annoyance if you, or your luggage, obstruct them.
However, if you are stuck, Londoners are surprisingly helpful, so just ask. Even if the first person is in a hurry, doesn't know or doesn't speak English, you will soon find someone who will go out of his/her way to help.