how to add another post page

Hi
I recently modified (and purchased, thank you!) the paalam theme, and found it easy to work with.
I am now trying to add another page of posts.
If you look at my site:
https://blog.goldassets.co.uk
Basically what I want is a button on the header bar (‘home’, ‘about’ and (new)’market updates’) which would be the same as the index page except only displaying posts from the market updates category.
I gather this is rather advanced, but I am confident ?? I just need a few pointers of where to start.
I think I need to edit the functions in query.php to change the_post() function to be something like the_market_post() and then add variables to the WP_Query class to contain only the market posts…

Am I on the right track? Any suggestions/examples?

TIA
Ed