This site defines a logarithm and has practice worksheets.
http://www.math.utah.edu/~alfeld/math/log.html
These sites give an overview of logarithm:
http://hyperphysics.phy-astr.gsu.edu/hbase/logm.html
Review logarithms at this interactive site:
http://academic.pgcc.edu/~ssinex/rad_decay.pdf