"Someone took my bicycle."

Translation: Někdo vzal moje kolo.

[JBHayven]

Does "brat si" mean only "to marry" or can it also be used in a context of taking something for yourself/away?

[JanLyko]

Both. For example "Beru si dvě vejce" (I am taking two eggs).

[kacenka9]

There would be a little different meaning though. Vzít is perfective and brát imperfective verb. Somebody took your bike (vzít) and now they have it. Brát in past tense would have to be translated as "somebody was taking my bike". Either regularly borrowing it or you saw them as they were taking it.

[JBHayven]

Yeah, I just used the imperfective in the question because it seemed better to me. What I actually tried here was "Někdo vzal si moje kolo", which was rejected, hence the question.

[OndraSl]

Ironically, "Někdo si vzal moje kolo." is something I'd rather use as opposed to "Někdo vzal moje kolo." as a Czech person. Depending on the part of the country you come from, I guess.