Learn French: Où est le témoin du crime ? - Where is the witness of the crime?

Traditionally, témoin is always masculine (un témoin or le témoin), regardless of whether the person is male or female. You might occasionally see la témoin in modern French, but using le témoin is always correct.

Why do we use du instead of just de here?

Du is a mandatory contraction. In French, you can never say de le. Because crime is a masculine noun, saying "of the crime" means combining de (of) and le (the), which automatically shrinks into du.

How important is the accent on the word où?

It is very important for your written French! With the grave accent, où means "where". Without the accent, ou means "or". They sound completely identical when spoken, so the accent is the only way to tell them apart in writing.

Does un crime mean exactly the same thing as "a crime" in English?

They are very close, but in French, un crime specifically refers to a very serious or violent offense, like murder. For lesser crimes, like theft or vandalism, the French usually use the word un délit.