Have you ever watched a movie where there are different characters having different parts of the story, and not until nearly the end of the movie do you get to see how it all comes together? Tale of Two Cities is like that. It's okay to not understand why you are reading about these people, or why the story jumps from one seemingly unrelated event to another.
If you just read along, like you are watching the movie, there will be people in each chapter. You will have a feeling about them, whether you like them or not. Whether you think they are nice or not, good or bad. Then at the end, Dickens ties all the stories together, and some people have good things happen to them, and some have bad things happen to them, and then you get to decide how you feel about that.
So do try to read the book, and don't worry about understanding the plot for now.
The two cities are London and Paris. All the parts of the story happen in one or the other of those two places. The story is set right around the time of the beginning of the French Revolution. The reader will be expected to know that the French revolution was a revolt of peasants against the ruling class, and you need to know what a guillotine is. Partly, this is just small story, about a few people and what happened in their lives. There is a kind of a love story. But it is also about the bigger picture of the revolution, because when the revolution happened, these people's lives got caught up in it.