See where high frequency currents flow
Skim through Skin Effect and Proximity Effect

Skim through 17.4.4.1.1 Trace Antennas (pg. 15)
Look at how a pc board trace changes impedance with frequency
and why you should minimize loop area?
Look over Routing & Bypass 17.8.4-17.8.5 (pg. 30) to
get an idea of the rules for good high speed pc board layout.
Circuit Board Layout Techniques

Skim through REDUCING CAPACITIVE & MAGNETICALLY-COUPLED NOISE (Page 5)
and A REVIEW OF SHIELDING CONCEPTS (Page 8-10)
and how the size of a hole in a shield relates to the frequency
that can enter through the hole (Eq. 6 page 10)
EMI, RFI, and Shielding Concepts

Various skin depths
Skin depth graph
Magnetic Shielding Basics
Round wire resistance calculator