A lot of documentation about Isabelle is available.
Have a look at this tutorial for instance and later consult the Isar Reference Manual for more advanced features, or better ask questions in this stream!
Make sure to follow the link to the tutorial Anthony gave, there is an old tutorial available: https://isabelle.in.tum.de/doc/tutorial.pdf. which will pop up first if you google "Isabelle tutorial". It is still useful but it's outdated and it's much better to start with the new prog-prove tutorial :)
Last updated: Dec 07 2023 at 08:19 UTC