Roles and permissions

Hi,

I realise I posted this originally in the wrong forum and had the wrong slant on the question - MyMoodle probably isn't my best option.

I want Role A, after logging in, to see a list of categories and have access too all courses underneath those categories.

I want Role B, after logging in, to see only the course they are logged into.

I want both roles to only have student rights within the courses.

Currently, both roles can see everything. I've added Role A to the categories as students and Role B to the courses as students - so the restrictions are fine but I don't want Role B to see what they can't have access to.

How do I restrict Role B to just seeing their single course? How do I do that without Role A losing their category view?

I appreciate your help.

Cariad