Complete the senior student's slightly dramatic mentoring speech to a freshman.

Let me give you _____ before you take Professor Smith's notoriously difficult physics exam.

The correct answer is a piece of advice.

In English, advice is always an uncountable noun. It cannot be pluralized (no "advices") and cannot be used with the indefinite article "an".

To make an uncountable noun countable so you can refer to a single item, we use a partitive phrase like a piece of. ("Some advice" would also be correct, but "some advices" is grammatically incorrect).