"When do you take your shower?"
Translation:Quand prends-tu ta douche ?
I'm no expert, but I believe 'proper' French requires the question word "quand" to come first. After that, you can choose to invert the question (formal) or to not (informal).
It's possible that French slang might allow you to move "quand". I recall Sitesurf giving an example of this with "ou".
No, it's not absolutely necessary to be understood. You can put your chosen determiner in front of "douche" and get your point across.
As for the translation, it actually maps quite directly here.
- prends ta douche = take your shower
- prends une douche = take a shower
- prends la douche = take the shower
You can also ask yourself in English why we say "when do you take your shower" as opposed to just "when do you take a shower."