Read more problem when using different pages for different categories

I use WordPress on my own external php pages and I don’t use any templates.

I have different categories on different pages who all looks different from eachother.
My problem is when I click on “read more” to read a full post the post is always shown on the indexpage of my homepage. Instead I want them to show up on the page of that category.

For example if I click on a “read more” link in category1.php I want the post to show up on that page and not on index.php.

Would be very greatful for a solution to this.