Start small and work up. Pick two different note values (crotchet and quaver) and practice reading short examples that use just those two.
Something like this might help:
http://readsheetmusic.info/reading_rhythms_exercises.shtml
Then try two different note values (quavers and semiquavers) and then introduce crotchets, quavers and semiquavers in the later stages.
Reading is a really valuable skill, and it can take a while to get to grips with it. The trick is to do a little and often (well, as much as you can, but certainly often).
Hope this helps.