There is a lot of material about cosmology on the Web, but caveat emptor! Here are some reliable sources (in ~increasing order of difficulty):
Relativity FAQ (see in particular the section on ``General Relativity and Cosmology'')
A brief history of cosmology (... and other entries on the St Andrews maths archive)
Cosmic journey: a history of scientific cosmology (AIP)
Universe 101: Big Bang Theory (NASA)
Cosmic microwave background FAQ (Douglas Scott, UBC)
Cosmology tutorial (Ned Wright, UCLA)
An introduction to the cosmic microwave background (Wayne Hu, Chicago U)
Feel free to email me
if you have a specific question concerning my lectures