5S your space. For the most part you only need to worry about the first two and the last one.

First, sort, and set in place. This is kind of a big project and you might want to start with something small first, like your desk. I don't have any specific advice on how to get this done, but while this is a critical step it's not the one most ADHDers have an issue with.

When that's done:

Every time you pick something up, a before your put it back down, recite this mantra:

Don't put it down, put it away. This is sustain, and is generally where most ADHDers fail at organization.

Don't put it down. Put it away.