Your question is a bit too general.
Design guidelines for composite structures are much the same as for structures made of anything. For a laminated composite, if the matrix is polymer or ceramic, the 3-direction (through the thickness) is vulnerable to tension, and also impacts in this direction should be avoided.
For a polymer matrix composite degradation due to heat and moisture ingress must be allowed for.
For continous fibre reinforcement most of the fibres should be lined up with the main load directions if possible, but probably some fibres (some quidelines say a minimum of 10%) should be all primary directions (usually 0°, +45°, -45° and 90°).
Generally it is a good idea to make monolithic laminates symmetric.
The fiber strength is generally supplied by the material suppliers, though often the finished composite material properties are generated from testing by the people creating the material specification.
Calculating the strength of a laminate is often done using Classical Laminate Theory (CLT), but for aerospace primary structure there is a lot of testing and CLT is not usually used.
for a good introduction to these things.
You mention a 'rail' in the subject. If you can tell us a bit about the rail we may be able to help more.
Come back when you have a more specific design question.