How to define a if-condition?

It may be considered obsolete because there exist more modern variants and the package was not updated for a long time.

See for example http://www.ctan.org/pkg/etoolbox which uses e-TeX features.

Apart from the macros in ifthenelse being fragile, if you can handle that, you can just use them, no problem.

I prefer

\def\something{} %% set to true
%% or:
\let\something\undefined %% set to false

\ifdefined\something
  True
\else
  False
\fi

to avoid the ifthen package.

The advantage of not using a package is that this approach can be used in files that are just \input by large document (for example TikZ images), when it cannot be guaranteed that a certain package is loaded.