Learn Turkish: Evde kal.

In Turkish, giving a direct command to one person (the informal "you") is incredibly simple: you just use the bare root of the verb. The dictionary form for "to stay" is kalmak. When you drop the -mak ending, you are left with the root kal, which gives the direct command "Stay!".

Why is it evde instead of just ev?

Ev simply means "home" or "house". To say "at home", you need to add the locative suffix -de (which means "at", "on", or "in" depending on the word). So, ev + -de becomes evde.

Do I need to include sen (you) at the beginning of the sentence?

You don't have to! In Turkish, the subject is usually dropped because it is understood from the verb. Even though kal has no personal endings attached to it, this bare root form is specifically used for commanding sen (the singular, informal "you"). Saying Sen evde kal is perfectly correct, but it sounds like you are emphasizing "You stay at home" (as opposed to someone else staying).