The creation of elements in the Big Bang has a direct effect on the elements within stars. Not surprisingly then, stars are largely made of up of hydrogen and helium. The exact numbers vary from star to star, but they are typically 73% hydrogen and 25% helium. That leaves 2% for other elements. The two most common "other" elements are carbon and oxygen. Older stars have heavier elements than younger stars.

Lighter elements in stars began fusing to create heavier elements, producing all the other naturally occurring elements. At high temperatures and pressures, heavier elements are able to form. For example, helium fuses with carbon to make oxygen, and helium fuses with oxygen to make neon.

The following video explains how elements are created in stars.


Sources
http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_mainsequence.html and Teachers' Domain, The Elements: Forged in Stars, published December 17, 2005, retrieved on July 14, 2010,
http://www.teachersdomain.org/resource/ess05.sci.ess.eiu.fusion/