Я принимаю душ.

Я пойду в душ means I'm going to have a shower (now) By the way in English we don't say 'take' a shower, but HAVE a shower

Я принимаю душ. But Svetlana's suggenstion need some commentary. "Я в душе" (I am in the shower) сan be used only in the sense "I am takING shower [at the moment]". "Я принимаю душ" also works. For "I take a shower [each morning]" 'Я принимаю душ [каждое утро]' is the only option. In Past: 'я был в душе" (I was in the shower) may mean 'I was taking a shower [at some moment'. Also it can be used in a question: 1."ты БЫЛ в душе?" ('have you been there?' that is 'have you took it already?'). 2. "ТЫ был в душе?" (was it you?) 3. "ты был в душе?" ('where you were?') Similarly it may occur as an answer (a confirmation), again with the contrastive stress for 1 and 2. It is the same pattern as "ты был в Японии?" except it allows 'бывал' ('have EVER been'). Now, as to 'more common'... They are slightly different in meaning:) "why did not you answer me?" "I was in the shower, I didn't hear:(". "What are you doing now?" "I'm taking/having a shower" As to questions, I mostly use 'принимал/принял': "ты [уже] приняла душ?" 'have you took a shower already?' (I know she planned it 15 mins ago) "ты [сегодня] принимал душ?" (I'm curious if he have had it today.). "Ты принимал душ?" (I called him 15 mins ago, he didn't answer. I'm curious why.). Don't read much into it, it is individual habits.

Я принимаю душ

You can also say: "я в душе" It's more common. If you need to say in the past, then "я был в душе"