There is an old dog house I found that is being junked (I think i'll use it as a temporary chicken coop) I don't know how clean it is I mean If anything died in it or something So I was wondering what's the best way to clean it?
Unless it's had chickens or rabbits in it, there is unlikely to be any problem.
If you are really concerned, though, scrub it real good with a hard-bristled brush and soapy hot water (or pressure-wash), then let it air dry very thoroughly.
If you are SUPER crazily concerned there are disinfectants that can be used, but to my way of thinking, "why"
Do make sure you add ventilation. Doghouses are pretty much never ventilated even remotely sufficiently for chickens.