Go to draw.io
Note that this is the full name of the site - there is no 'www' at the beginning. If you type just 'draw.io' into your browser address field it will be redirected correctly.
Choose where to save diagrams
The first decision you are asked to make is where your diagrams should be stored. Choose the appropriate option. I recommend unticking the 'remember' box so that you can change your mind easily.
Create a new diagram
There are some basic diagrams to start from, but often it is easiest to start from a blank. That is what you should choose here. You should also choose a name for your diagram at this stage. It is possible to do it later, but easy to forget.
This takes you to the drawing canvas with shape palettes on the left and options on the right.