For a start, make entirely sure you have found a bug. Double-check with books about TeX, LaTeX, or whatever you're using; compare what you're seeing against the other answers above; ask every possible person you know who has any TeX-related expertise. The reasons for all this caution are various.
If you've found a bug in TeX itself, you're a rare animal indeed. Don Knuth is so sure of the quality of his code that he offers real money prizes to finders of bugs; the cheques he writes are such rare items that they are seldom cashed. If you think you have found a genuine fault in TeX itself (or Metafont, or the CM fonts, or the TeXbook), don't immediately write to Knuth, however. He only looks at bugs once or twice a year, and even then only after they are agreed as bugs by a small vetting team. In the first instance, contact Barbara Beeton at the AMS (bnb@math.ams.org), or contact TUG.
If you've found a bug in LaTeX2e, report it using mechanisms supplied in the LaTeX distribution.
If you've found a bug in LaTeX 2.09, or some other such unsupported software, there's not a lot you can do about it. You may find help or de facto support on a newsgroup such as comp.tex.tex or on a mailing list such as texhax@tex.ac.uk, but posting non-bugs to any of these forums can lay you open to ridicule! Otherwise you need to go out and find yourself a willing TeX-consultant - TUG maintains a register of TeX consultants.