How to create a dynamic website using python connected to a database
You could use a database, but that doesn't seem necessary for what you described.
It would be reasonable to build the database as you described.  Look into SQL for doing so.  You can download a package XAMPP that will give you pretty much everything you need for that.  This is easily scalable to hundreds of thousands of entries - that's what databases are for.
If your example of stock prices is actually what you are trying to show, however, this is completely unnecessary as there are already plenty of databases that have this data and will allow you to query them.  What you would really want in this scenario is an API.  Alpha Vantage is a free service that will serve you data on stock prices, and has plenty of documentation to help you get it set up with python.
I would structure the project like this:
Use the python library Flask to set up the back end.
In addition to instantiating the Flask app, instantiate the Alpha Vantage class as well (you will need to pip install both of these).
In one of the routes you declare under Flask, use the Alpha Vantage api to get the data you need and simply display it to the screen.
If I am assuming you are a complete beginner, one or more of those steps may not make sense to you, in which case take them one at a time.  Start by learning how to build a basic Flask app, then look at the API.
YouTube is your friend for both of these things.
I would like to create a website where I show some text but mainly dynamic data in tables and plots. Let us assume that the user can choose whether he wants to see the DAX or the DOW JONES prices for a specific timeframe. I guess these data I have to store in a database. As I am not experienced with creating websites, I have no idea what the most reasonable setup for this website would be. 

Would it be reasonable for this example to choose a database where every row corresponds of 9 fields, where the first column is the timestamp (lets say data for every minute), the next four columns correspond  to the high, low, open, close price of DAX for this timestamp and columns 5 to 9 correspond to high, low, open, close price for DOW JONES?
Could this be scaled to hundreds of columns with a reasonable speed
of the database?
Is this an efficient implementation?
When this website is online, you can choose whether you want to see DAX or DOW JONES prices for a specific timeframe. The corresponding data would be chosen via python from the database and plotted in the graph. Is this the general idea how this will be implemented?
To get the data, I can run another python script on the webserver to dynamically collect the desired data and write them in the database?

As a total beginner with webhosting (is this even the right term?) it is very hard for me to ask precise questions. I would be happy if I could find out whats the general structure I need to create the website, the database and the connection between both. I was thinking about amazon web services.