I have not played Pokemon since Diamond/Pearl but recently pre-bought Pokemon Violet. Any advice?

As the text says. I have no idea what to do or expect.

I have very briefly played Pokemon Moon one time and I remembered finding Magikarps on this lake and getting annoyed at them spawning. I told my friends about it the other day and their jaw dropped. Apparently it's rare to find them but I didn't know that so seriously. Please help so I don't make that same mistake again.

Thank u lol